Change description of relative field in input drag event.

(cherry picked from commit 76c50928a1422cfddf5f0c3a4045454e080d4c07)

doc/classes/InputEventScreenDrag.xml

@@ -19,7 +19,7 @@
 			The drag position.
 		</member>
 		<member name="relative" type="Vector2" setter="set_relative" getter="get_relative" default="Vector2( 0, 0 )">
-			The drag position relative to its start position.
+			The drag position relative to the previous position (position at the last frame).
 		</member>
 		<member name="speed" type="Vector2" setter="set_speed" getter="get_speed" default="Vector2( 0, 0 )">
 			The drag speed.