Add trailing path separator to SDL_GetUserFolder()

include/SDL3/SDL_filesystem.h

@@ -224,6 +224,9 @@ typedef enum SDL_Folder
  * Note that the function is expensive, and should be called once at the
  * beginning of the execution and kept for as long as needed.
  *
+ * The returned path is guaranteed to end with a path separator ('\\' on
+ * Windows, '/' on most other platforms).
+ *
  * The returned value is owned by the caller and should be freed with
  * SDL_free().
  *