GLES2: Add comments around EXT_shader_texture_lod check

To avoid reintroducing bugs as I did in #26928 and #26932.

texture2DLodEXT and textureCubeLodEXT are only for the fragment shader with
https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_shader_texture_lod.txt
In the vertex shader, texture2DLod and textureCubeLod are built-in.