Merge pull request #2841 from skyace65/spatialupdate

Update spatial material page for Godot 3.2

tutorials/3d/spatial_material.rst

@@ -40,6 +40,12 @@ specified otherwise. The main settings that enable transparency are:
 * Blend mode set to other than "Mix"
 * Enabling distance or proximity fade
 
+Use Shadow to Opacity
+~~~~~~~~~~~~~~~~~~~~~
+
+Lighting modifies the alpha so shadowed areas are opaque and non-shadowed
+areas are transparent. Useful for overlaying shadows onto a camera feed in AR.
+
 Unshaded
 ~~~~~~~~