filesystem: Check SDL_GetPrefPath parameters at the higher level.

...so the backends don't have to do it.

Also added a stern warning about `org` being omitted, but leaving it as
allowed so as not to break existing apps (more than they are already broken,
at least).

Fixes #13322.

include/SDL3/SDL_filesystem.h

@@ -134,6 +134,12 @@ extern SDL_DECLSPEC const char * SDLCALL SDL_GetBasePath(void);
  * - ...only use letters, numbers, and spaces. Avoid punctuation like "Game
  *   Name 2: Bad Guy's Revenge!" ... "Game Name 2" is sufficient.
  *
+ * Due to historical mistakes, `org` is allowed to be NULL or "". In such
+ * cases, SDL will omit the org subdirectory, including on platforms where it
+ * shouldn't, and including on platforms where this would make your app fail
+ * certification for an app store. New apps should definitely specify a real
+ * string for `org`.
+ *
  * The returned path is guaranteed to end with a path separator ('\\' on
  * Windows, '/' on most other platforms).
  *