The warning message had an incorrect text and a wrong description in the manual.

It's issued when a texture exceeds 2048 pixels or a cubemap exceeds 256x256, indicating that you'll get problems with that texture on some older systems.

The warning message still exists, but will only be issued with a warn_level of 2 or above. And textures that are too big will now automatically be reduced in size, so the error won't be serious.