Restore buggy texture overlay modifier (#14852)

Required because of backwards compatibility.

doc/lua_api.md

@@ -456,6 +456,10 @@ i.e. without gamma-correction.
 
 Textures can be overlaid by putting a `^` between them.
 
+Warning: If the lower and upper pixels are both semi-transparent, this operation
+does *not* do alpha blending, and it is *not* associative. Otherwise it does
+alpha blending in srgb color space.
+
 Example:
 
     default_dirt.png^default_grass_side.png