Added documentation for AppBar actions (#109351)

* Added documentation for AppBar actions
Co-authored-by: Qun Cheng <quncheng@google.com>

packages/flutter/lib/src/material/app_bar.dart

@@ -343,6 +343,9 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
   /// The [actions] become the trailing component of the [NavigationToolbar] built
   /// by this widget. The height of each action is constrained to be no bigger
   /// than the [toolbarHeight].
+  ///
+  /// To avoid having the last action covered by the debug banner, you may want
+  /// to set the [MaterialApp.debugShowCheckedModeBanner] to false.
   /// {@endtemplate}
   ///
   /// {@tool snippet}