Heavy work on improving category documentation.

Still more to go!

Reference Issue #9440.

include/SDL3/SDL_main.h

@@ -35,6 +35,13 @@
  *
  * SDL will take care of platform specific details on how it gets called.
  *
+ * This is also where an app can be configured to use the main callbacks,
+ * via the SDL_MAIN_USE_CALLBACKS macro.
+ *
+ * This is a "single-header library," which is to say that including this
+ * header inserts code into your program, and you should only include it
+ * once in most cases. SDL.h does not include this header automatically.
+ *
  * For more information, see:
  *
  * https://wiki.libsdl.org/SDL3/README/main-functions