Add clarification in Animation's listener API docs (#33363)

packages/flutter/lib/src/animation/animation.dart

@@ -63,6 +63,9 @@ abstract class Animation<T> extends Listenable implements ValueListenable<T> {
 
   /// Stop calling the listener every time the value of the animation changes.
   ///
+  /// If `listener` is not currently registered as a listener, this method does
+  /// nothing.
+  ///
   /// Listeners can be added with [addListener].
   @override
   void removeListener(VoidCallback listener);
@@ -74,6 +77,9 @@ abstract class Animation<T> extends Listenable implements ValueListenable<T> {
 
   /// Stops calling the listener every time the status of the animation changes.
   ///
+  /// If `listener` is not currently registered as a status listener, this
+  /// method does nothing.
+  ///
   /// Listeners can be added with [addStatusListener].
   void removeStatusListener(AnimationStatusListener listener);