SDL_filesystem.h: Add a `\sa` from SDL_GetUserFolder to SDL_Folder.

Reference https://github.com/libsdl-org/sdlwiki/issues/522

include/SDL3/SDL_filesystem.h

@@ -237,6 +237,8 @@ typedef enum SDL_Folder
  *          folder, or NULL if an error happened.
  *
  * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_Folder
  */
 extern DECLSPEC char *SDLCALL SDL_GetUserFolder(SDL_Folder folder);