include: add comment documenting the change(s) to SDL_opengl_glext.h

include/SDL_opengl_glext.h

@@ -1,3 +1,6 @@
+// SDL modified the include guard to be compatible with Mesa and Apple include guards:
+// - Mesa uses: __gl_glext_h_
+// - Apple uses: __glext_h_
 #if !defined(__glext_h_) && !defined(__gl_glext_h_)
 #define __glext_h_ 1
 #define __gl_glext_h_ 1