Mark GlobalScope as delicate API and rewrite coroutine basics doc without it (#2637)

Also, remove the tutorial variant of the basic coroutine introduction as it essentially duplicates the content of the
basic coroutine introduction, albeit with "how to create a project" additional information, which does not seem to be appropriate in this section at all.

Fixes #2122

Co-authored-by: Vsevolod Tolstopyatov <qwwdfsad@gmail.com>
Co-authored-by: Andrey Polyakov <koshachy@gmail.com>

Author: 3 people

docs/images/new-mvn-project-jvm.png

@@ -24,7 +24,7 @@ suspend fun doSomethingUsefulTwo(): Int {
 
 What do we do if we need them to be invoked _sequentially_ — first `doSomethingUsefulOne` _and then_ 
 `doSomethingUsefulTwo`, and compute the sum of their results? 
-In practice we do this if we use the result of the first function to make a decision on whether we need 
+In practice, we do this if we use the result of the first function to make a decision on whether we need 
 to invoke the second one or to decide on how to invoke it.
 
 We use a normal sequential invocation, because the code in the coroutine, just like in the regular 
@@ -190,18 +190,26 @@ standard `lazy` function in cases when computation of the value involves suspend
 ## Async-style functions
 
 We can define async-style functions that invoke `doSomethingUsefulOne` and `doSomethingUsefulTwo`
-_asynchronously_ using the [async] coroutine builder with an explicit [GlobalScope] reference.
+_asynchronously_ using the [async] coroutine builder using a [GlobalScope] reference to 
+opt-out of the structured concurrency.
 We name such functions with the 
 "...Async" suffix to highlight the fact that they only start asynchronous computation and one needs
 to use the resulting deferred value to get the result.
 
+> [GlobalScope] is a delicate API that can backfire in non-trivial ways, one of which will be explained
+> below, so you must explicitly opt-in into using `GlobalScope` with `@OptIn(DelicateCoroutinesApi::class)`. 
+>
+{type="note"}
+
 ```kotlin
 // The result type of somethingUsefulOneAsync is Deferred<Int>
+@OptIn(DelicateCoroutinesApi::class)
 fun somethingUsefulOneAsync() = GlobalScope.async {
     doSomethingUsefulOne()
 }
 
 // The result type of somethingUsefulTwoAsync is Deferred<Int>
+@OptIn(DelicateCoroutinesApi::class)
 fun somethingUsefulTwoAsync() = GlobalScope.async {
     doSomethingUsefulTwo()
 }
@@ -236,10 +244,12 @@ fun main() {
 }
 //sampleEnd
 
+@OptIn(DelicateCoroutinesApi::class)
 fun somethingUsefulOneAsync() = GlobalScope.async {
     doSomethingUsefulOne()
 }
 
+@OptIn(DelicateCoroutinesApi::class)
 fun somethingUsefulTwoAsync() = GlobalScope.async {
     doSomethingUsefulTwo()
 }
@@ -272,9 +282,9 @@ Completed in 1085 ms
 {type="note"}
 
 Consider what happens if between the `val one = somethingUsefulOneAsync()` line and `one.await()` expression there is some logic
-error in the code and the program throws an exception and the operation that was being performed by the program aborts. 
+error in the code, and the program throws an exception, and the operation that was being performed by the program aborts. 
 Normally, a global error-handler could catch this exception, log and report the error for developers, but the program 
-could otherwise continue doing other operations. But here we have `somethingUsefulOneAsync` still running in the background,
+could otherwise continue doing other operations. However, here we have `somethingUsefulOneAsync` still running in the background,
 even though the operation that initiated it was aborted. This problem does not happen with structured
 concurrency, as shown in the section below.
 
@@ -293,7 +303,7 @@ suspend fun concurrentSum(): Int = coroutineScope {
 }
 ```
 
-This way, if something goes wrong inside the code of the `concurrentSum` function and it throws an exception,
+This way, if something goes wrong inside the code of the `concurrentSum` function, and it throws an exception,
 all the coroutines that were launched in its scope will be cancelled.
 
 <!--- CLEAR -->
@@ -403,4 +413,4 @@ Computation failed with ArithmeticException
 [CoroutineScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope/index.html
 [_coroutineScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/coroutine-scope.html
 
-<!--- END -->
+<!--- END -->

docs/topics/composing-suspending-functions.md

@@ -65,9 +65,8 @@ context of the main `runBlocking` coroutine which runs in the `main` thread.
 [Dispatchers.Unconfined] is a special dispatcher that also appears to run in the `main` thread, but it is,
 in fact, a different mechanism that is explained later.
 
-The default dispatcher that is used when coroutines are launched in [GlobalScope]
-is represented by [Dispatchers.Default] and uses a shared background pool of threads,
-so `launch(Dispatchers.Default) { ... }` uses the same dispatcher as `GlobalScope.launch { ... }`.
+The default dispatcher that is used when no other dispatcher is explicitly specified in the scope.
+It is represented by [Dispatchers.Default] and uses a shared background pool of threads.
 
 [newSingleThreadContext] creates a thread for the coroutine to run. 
 A dedicated thread is a very expensive resource. 
@@ -303,8 +302,14 @@ the [Job] of the new coroutine becomes
 a _child_ of the parent coroutine's job. When the parent coroutine is cancelled, all its children
 are recursively cancelled, too. 
 
-However, when [GlobalScope] is used to launch a coroutine, there is no parent for the job of the new coroutine.
-It is therefore not tied to the scope it was launched from and operates independently.
+However, this parent-child relation can be explicitly overriden in one of two ways:
+
+1. When a different scope is explicitly specified when launching a coroutine (for example, `GlobalScope.launch`), 
+   then it does not inherit a `Job` from the parent scope.
+2. When a different `Job` object is passed as the context for the new coroutine (as show in the example below),
+   then it overrides the `Job` of the parent scope.
+   
+In both cases, the launched coroutine is not tied to the scope it was launched from and operates independently.
 
 ```kotlin
 import kotlinx.coroutines.*
@@ -313,9 +318,9 @@ fun main() = runBlocking<Unit> {
 //sampleStart
     // launch a coroutine to process some kind of incoming request
     val request = launch {
-        // it spawns two other jobs, one with GlobalScope
-        GlobalScope.launch {
-            println("job1: I run in GlobalScope and execute independently!")
+        // it spawns two other jobs
+        launch(Job()) { 
+            println("job1: I run in my own Job and execute independently!")
             delay(1000)
             println("job1: I am not affected by cancellation of the request")
         }
@@ -343,7 +348,7 @@ fun main() = runBlocking<Unit> {
 The output of this code is:
 
 ```text
-job1: I run in GlobalScope and execute independently!
+job1: I run in my own Job and execute independently!
 job2: I am a child of the request coroutine
 job1: I am not affected by cancellation of the request
 main: Who has survived request cancellation?
@@ -659,7 +664,6 @@ that should be implemented.
 [async]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/async.html
 [CoroutineScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope/index.html
 [Dispatchers.Unconfined]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-dispatchers/-unconfined.html
-[GlobalScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-global-scope/index.html
 [Dispatchers.Default]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-dispatchers/-default.html
 [newSingleThreadContext]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/new-single-thread-context.html
 [ExecutorCoroutineDispatcher.close]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-executor-coroutine-dispatcher/close.html
@@ -678,4 +682,4 @@ that should be implemented.
 [ensurePresent]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/java.lang.-thread-local/ensure-present.html
 [ThreadContextElement]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-thread-context-element/index.html
 
-<!--- END -->
+<!--- END -->