wikiheaders: bridge wiki Category docs to the headers!

Did an initial cleanup on the headers and wrote a few pieces of documentation,
but this needs more work to fill out the documentation.

include/SDL3/SDL_opengl.h

@@ -19,17 +19,11 @@
   3. This notice may not be removed or altered from any source distribution.
 */
 
-/**
- *  \file SDL_opengl.h
+/*
+ * This is a simple file to encapsulate the OpenGL API headers.
  *
- *  This is a simple file to encapsulate the OpenGL API headers.
- */
-
-/**
- *  \def NO_SDL_GLEXT
- *
- *  Define this if you have your own version of glext.h and want to disable the
- *  version included in SDL_opengl.h.
+ * Define NO_SDL_GLEXT if you have your own version of glext.h and want
+ * to disable the version included in SDL_opengl.h.
  */
 
 #ifndef SDL_opengl_h_