Make syncShutdown unavailable from async (#667)

* Make syncShutdown unavailable from async

Motivation

syncShutdown can cause unbounded thread blocking, we shouldn't allow it
in concurrent code.

Modification

Mark syncShutdown unavailable from async.

Result

Users are warned if they try to syncShutdown in an async context

* Only noasync on 5.7

Author: Lukasa

README.md

@@ -63,7 +63,7 @@ httpClient.get(url: "https://apple.com/").whenComplete { result in
 }
 ```
 
-You should always shut down `HTTPClient` instances you created using `try httpClient.syncShutdown()`. Please note that you must not call `httpClient.syncShutdown` before all requests of the HTTP client have finished, or else the in-flight requests will likely fail because their network connections are interrupted.
+You should always shut down `HTTPClient` instances you created using `try httpClient.shutdown()`. Please note that you must not call `httpClient.shutdown` before all requests of the HTTP client have finished, or else the in-flight requests will likely fail because their network connections are interrupted.
 
 ### async/await examples
 

Sources/AsyncHTTPClient/Docs.docc/index.md

@@ -71,7 +71,7 @@ httpClient.get(url: "https://apple.com/").whenComplete { result in
 }
 ```
 
-You should always shut down ``HTTPClient`` instances you created using ``HTTPClient/syncShutdown()``. Please note that you must not call ``HTTPClient/syncShutdown()`` before all requests of the HTTP client have finished, or else the in-flight requests will likely fail because their network connections are interrupted.
+You should always shut down ``HTTPClient`` instances you created using ``HTTPClient/shutdown()-96ayw()``. Please note that you must not call ``HTTPClient/shutdown()-96ayw()`` before all requests of the HTTP client have finished, or else the in-flight requests will likely fail because their network connections are interrupted.
 
 ### async/await examples
 

Sources/AsyncHTTPClient/HTTPClient.swift

@@ -139,17 +139,29 @@ public class HTTPClient {
                 """)
             case .upAndRunning:
                 preconditionFailure("""
-                Client not shut down before the deinit. Please call client.syncShutdown() when no \
+                Client not shut down before the deinit. Please call client.shutdown() when no \
                 longer needed. Otherwise memory will leak.
                 """)
             }
         }
     }
 
+    #if swift(>=5.7)
     /// Shuts down the client and `EventLoopGroup` if it was created by the client.
+    ///
+    /// This method blocks the thread indefinitely, prefer using ``shutdown()-96ayw``.
+    @available(*, noasync, message: "syncShutdown() can block indefinitely, prefer shutdown()", renamed: "shutdown()")
+    public func syncShutdown() throws {
+        try self.syncShutdown(requiresCleanClose: false)
+    }
+    #else
+    /// Shuts down the client and `EventLoopGroup` if it was created by the client.
+    ///
+    /// This method blocks the thread indefinitely, prefer using ``shutdown()-96ayw``.
     public func syncShutdown() throws {
         try self.syncShutdown(requiresCleanClose: false)
     }
+    #endif
 
     /// Shuts down the client and `EventLoopGroup` if it was created by the client.
     ///