Tidy up @InternalCoroutinesApi documentation (#4086)

* @suppresses where necessary until Kotlin/dokka#1861 is fixed
* Add @MustBeDocumented in case if it's leaked into public documentation

Author: qwwdfsad

kotlinx-coroutines-core/common/src/Annotations.kt

@@ -80,6 +80,7 @@ public annotation class ObsoleteCoroutinesApi
  * `kotlinx.coroutines`, because their signatures and semantics will change between future releases without any
  * warnings and without providing any migration aids.
  */
+@MustBeDocumented
 @Retention(value = AnnotationRetention.BINARY)
 @Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION, AnnotationTarget.TYPEALIAS, AnnotationTarget.PROPERTY)
 @RequiresOptIn(

kotlinx-coroutines-core/common/src/CoroutineExceptionHandler.kt

@@ -11,6 +11,8 @@ import kotlin.coroutines.*
  * If there is [CoroutineExceptionHandler] in the context, then it is used. If it throws an exception during handling
  * or is absent, all instances of [CoroutineExceptionHandler] found via [ServiceLoader] and
  * [Thread.uncaughtExceptionHandler] are invoked.
+ *
+ * @suppress **This is internal API and it is subject to change.**
  */
 @InternalCoroutinesApi
 public fun handleCoroutineException(context: CoroutineContext, exception: Throwable) {

kotlinx-coroutines-core/common/src/channels/Channel.kt

@@ -475,6 +475,9 @@ public value class ChannelResult<out T>
         override fun toString(): String = "Closed($cause)"
     }
 
+    /**
+     * @suppress **This is internal API and it is subject to change.**
+     */
     @InternalCoroutinesApi
     public companion object {
         private val failed = Failed()

kotlinx-coroutines-core/common/src/intrinsics/Cancellable.kt

@@ -8,6 +8,8 @@ import kotlin.coroutines.intrinsics.*
 /**
  * Use this function to start coroutine in a cancellable way, so that it can be cancelled
  * while waiting to be dispatched.
+ *
+ * @suppress **This is internal API and it is subject to change.**
  */
 @InternalCoroutinesApi
 public fun <T> (suspend () -> T).startCoroutineCancellable(completion: Continuation<T>): Unit = runSafely(completion) {