Reference the BaseButton class explicitly in button class documentations

Users are sometimes confused as to the `pressed` property not being
visible in the Button class documentation. This is because `pressed`
is defined in BaseButton.

(cherry picked from commit 038baede5a)

doc/classes/LinkButton.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		This kind of button is primarily used when the interaction with the button causes a context change (like linking to a web page).
+		See also [BaseButton] which contains common properties and methods associated with this node.
 	</description>
 	<tutorials>
 	</tutorials>