Javadoc for background platform chnanels (flutter#34774)

Author: jiahaog

shell/platform/android/io/flutter/plugin/common/MethodChannel.java

@@ -173,8 +173,10 @@ public interface MethodCallHandler {
      * <p>Any uncaught exception thrown by this method will be caught by the channel implementation
      * and logged, and an error result will be sent back to Flutter.
      *
-     * <p>The handler is called on the platform thread (Android main thread). For more details see
-     * <a href="https://github.com/flutter/engine/wiki/Threading-in-the-Flutter-Engine">Threading in
+     * <p>The handler is called on the platform thread (Android main thread) by default, or
+     * otherwise on the thread specified by the {@link BinaryMessenger.TaskQueue} provided to the
+     * associated {@link MethodChannel} when it was created. See also <a
+     * href="https://github.com/flutter/flutter/wiki/The-Engine-architecture#threading">Threading in
      * the Flutter Engine</a>.
      *
      * @param call A {@link MethodCall}.
@@ -190,10 +192,7 @@ public interface MethodCallHandler {
    * Flutter methods provide implementations of this interface for handling results received from
    * Flutter.
    *
-   * <p>All methods of this class must be called on the platform thread (Android main thread). For
-   * more details see <a
-   * href="https://github.com/flutter/engine/wiki/Threading-in-the-Flutter-Engine">Threading in the
-   * Flutter Engine</a>.
+   * <p>All methods of this class can be invoked on any thread.
    */
   public interface Result {
     /**