Adds a @UsedByGodot mention in the Android Plugin docs. (#7151)

tutorials/platform/android/android_plugin.rst

@@ -125,8 +125,9 @@ Move the plugin configuration file (e.g: ``MyPlugin.gdap``) and, if any, its loc
 
 The Godot editor will automatically parse all ``.gdap`` files in the ``res://android/plugins`` directory and show a list of detected and toggleable plugins in the Android export presets window under the **Plugins** section.
 
-.. image:: img/android_export_preset_plugins_section.png
+In order to allow GDScript to communicate with your Java Singleton, you must annotate your function with ``@UsedByGodot``. The name called from GDScript must match the function name exactly. There is **no** coercing ``snake_case`` to ``camelCase``.
 
+.. image:: img/android_export_preset_plugins_section.png
 
 From your script::