Mention the stretching behavior of PopupPanel

(cherry picked from commit a2b088b1c1b79950212287fd4c8539a67cfcb56f)

doc/classes/PopupPanel.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		Class for displaying popups with a panel background. In some cases it might be simpler to use than [Popup], since it provides a configurable background. If you are making windows, better check [WindowDialog].
+		If any [Control] node is added as a child of this [PopupPanel], it will be stretched to fit the panel's size (similar to how [PanelContainer] works).
 	</description>
 	<tutorials>
 	</tutorials>