Improved MenuButton Description

I expanded description of MenueButton to explain how to create the items inside of the pop up as this is initially a mystery to many new people.

doc/classes/MenuButton.xml

@@ -4,7 +4,8 @@
 		Special button that brings up a [PopupMenu] when clicked.
 	</brief_description>
 	<description>
-		Special button that brings up a [PopupMenu] when clicked. That's pretty much all it does, as it's just a helper class when building GUIs.
+		Special button that brings up a [PopupMenu] when clicked. 
+		New items can be created inside this [PopupMenu] using [code]get_popup().add_item("My Item Name")[/code]. You can also create them directly from the editor. To do so, select the MenuButton node, then in the toolbar at the top of the 2D editor, click [b]Items[/b] then click [b]Add[/b] in the popup. You will be able to give each items new properties.
 	</description>
 	<tutorials>
 	</tutorials>