N3DS: Document the SDL_GetBasePath behaviour.

include/SDL_filesystem.h

@@ -60,6 +60,10 @@ extern "C" {
  * - `parent`: the containing directory of the bundle. For example:
  *   `/Applications/SDLApp/`
  *
+ * **Nintendo 3DS Specific Functionality**: This function returns "romfs"
+ * directory of the application as it is uncommon to store resources
+ * outside the executable. As such it is not a writable directory.
+ *
  * The returned path is guaranteed to end with a path separator ('\' on
  * Windows, '/' on most other platforms).
  *