Updated PopupMenu documentation after merge of #41640 - PopupMenu rework

doc/classes/PopupMenu.xml

@@ -5,6 +5,8 @@
 	</brief_description>
 	<description>
 		[PopupMenu] is a [Control] that displays a list of options. They are popular in toolbars or context menus.
+		The size of a [PopupMenu] can be limited by using [member Window.max_size]. If the height of the list of items is larger than the maximum height of the [PopupMenu], a [ScrollContainer] within the popup will allow the user to scroll the contents.
+		If no maximum size is set, or if it is set to 0, the [PopupMenu] height will be limited by its parent rect.
 	</description>
 	<tutorials>
 	</tutorials>