Mention how to handle cancel in ConfirmationDialog

doc/classes/ConfirmationDialog.xml

@@ -5,6 +5,10 @@
 	</brief_description>
 	<description>
 		Dialog for confirmation of actions. This dialog inherits from [AcceptDialog], but has by default an OK and Cancel button (in host OS order).
+		To get cancel action, you can use:
+		[codeblock]
+		get_cancel().connect("pressed", self, "cancelled")
+		[/codeblock].
 	</description>
 	<tutorials>
 	</tutorials>