Rename SDL_GetPath to SDL_GetUserFolder

The documentation has been edited accordingly, and certain parts have been clarified.

src/filesystem/unix/SDL_sysfilesystem.c

@@ -517,7 +517,7 @@ static char *xdg_user_dir_lookup (const char *type)
     return NULL;
 }
 
-char *SDL_GetPath(SDL_Folder folder)
+char *SDL_GetUserFolder(SDL_Folder folder)
 {
     const char *param = NULL;
     char *retval;