SDL_main.h: fix up SDL_main_func documentation to match SDL_main.

include/SDL3/SDL_main.h

@@ -440,8 +440,9 @@ extern SDLMAIN_DECLSPEC void SDLCALL SDL_AppQuit(void *appstate);
  *
  * \param argc an ANSI-C style main function's argc.
  * \param argv an ANSI-C style main function's argv.
- * \returns Generally 0 is considered successful program completion, small
- *          non-zero values are considered errors.
+ * \returns an ANSI-C main return code; generally 0 is considered successful
+ *          program completion, and small non-zero values are considered
+ *          errors.
  *
  * \since This datatype is available since SDL 3.0.0.
  */