filesystem: SDL_EnumerateDirectory() gives dirs with path seperators appended.

Fixes #11065.
Fixes #11427.

include/SDL3/SDL_filesystem.h

@@ -313,6 +313,9 @@ typedef enum SDL_EnumerationResult
  * terminate the enumeration early, and dictate the return value of the
  * enumeration function itself.
  *
+ * `dirname` is guaranteed to end with a path separator ('\\' on
+ * Windows, '/' on most other platforms).
+ *
  * \param userdata an app-controlled pointer that is passed to the callback.
  * \param dirname the directory that is being enumerated.
  * \param fname the next entry in the enumeration.