Added description for abs function in Rect2

(cherry picked from commit d1b3f36eab)

doc/classes/Rect2.xml

@@ -42,6 +42,7 @@
 			<return type="Rect2">
 			</return>
 			<description>
+				Returns a [code]Rect2[/code] with equivalent position and area, modified so that the top-left corner is the origin and [code]width[/code] and [code]height[/code] are positive.
 			</description>
 		</method>
 		<method name="clip">