Improved the AnimatedSprite docs; added description to speed_scale.

(cherry picked from commit ece6d37eef8f4476ba4891a44fb4d4c64435c470)

doc/classes/AnimatedSprite.xml

@@ -61,6 +61,7 @@
 			If [code]true[/code], the [member animation] is currently playing.
 		</member>
 		<member name="speed_scale" type="float" setter="set_speed_scale" getter="get_speed_scale">
+			The animation speed is multiplied by this value.
 		</member>
 	</members>
 	<signals>