Clarify when Client.close must be called (#1255)

pkgs/http/lib/src/client.dart

@@ -148,8 +148,11 @@ abstract interface class Client {
 
   /// Closes the client and cleans up any resources associated with it.
   ///
-  /// It's important to close each client when it's done being used; failing to
-  /// do so can cause the Dart process to hang.
+  /// Some clients maintain a pool of network connections that will not be
+  /// disconnected until the client is closed. This may cause programs using
+  /// using the Dart SDK (`dart run`, `dart test`, `dart compile`, etc.) to
+  /// not terminate until the client is closed. Programs run using the Flutter
+  /// SDK can still terminate even with an active connection pool.
   ///
   /// Once [close] is called, no other methods should be called. If [close] is
   /// called while other asynchronous methods are running, the behavior is