Mention an issue with infinite scroll on mirroring of Parallax layer

(cherry picked from commit 74df54a99e3e20d2e3378a7cf5c655a1727bb8c9)

doc/classes/ParallaxLayer.xml

@@ -15,6 +15,7 @@
 	<members>
 		<member name="motion_mirroring" type="Vector2" setter="set_mirroring" getter="get_mirroring" default="Vector2( 0, 0 )">
 			The ParallaxLayer's [Texture] mirroring. Useful for creating an infinite scrolling background. If an axis is set to [code]0[/code], the [Texture] will not be mirrored.
+			If the length of the viewport axis is bigger than twice the mirrored axis size, it will not repeat infinitely, as the parallax layer only draws 2 instances of the texture at any one time.
 		</member>
 		<member name="motion_offset" type="Vector2" setter="set_motion_offset" getter="get_motion_offset" default="Vector2( 0, 0 )">
 			The ParallaxLayer's offset relative to the parent ParallaxBackground's [member ParallaxBackground.scroll_offset].