Add to software skinning classref note about custom shaders

Added note to say that custom shaders operate on VERTEX after the transform with software skinning rather than before (as is the case with hardware skinning).

doc/classes/ProjectSettings.xml

@@ -1046,6 +1046,7 @@
 			If [code]true[/code], performs 2D skinning on the CPU rather than the GPU. This provides greater compatibility with a wide range of hardware, and also may be faster in some circumstances.
 			Currently only available when [member rendering/batching/options/use_batching] is active.
 			[b]Note:[/b] Antialiased software skinned polys are not supported, and will be rendered without antialiasing.
+			[b]Note:[/b] Custom shaders that use the [code]VERTEX[/code] built-in operate with [code]VERTEX[/code] position [i]after[/i] skinning, whereas with hardware skinning, [code]VERTEX[/code] is the position [i]before[/i] skinning.
 		</member>
 		<member name="rendering/2d/snapping/use_gpu_pixel_snap" type="bool" setter="" getter="" default="false">
 			If [code]true[/code], forces snapping of vertices to pixels in 2D rendering. May help in some pixel art styles.