Document need to include SDL_revision.h

If you want access to the SDL_REVISION define,
you must explicitly include this header.

Language like this was present in the SDL2 version,
but disappeared in SDL3, even though it's still true.

include/SDL3/SDL_revision.h

@@ -44,6 +44,9 @@
  * clue in debugging forensics and not something the app will parse in any
  * way.
  *
+ * SDL_revision.h must be included in your program explicitly if you want access
+ * to the SDL_REVISION constant.
+ *
  * \since This macro is available since SDL 3.2.0.
  */
 #define SDL_REVISION "Some arbitrary string decided at SDL build time"