Document the limit of 9 parts for paths in system.api.php

This affects both Drupal 7 & 8.
Since MENU_MAX_PARTS is set to 9 no router path can be over 9 parts long. This is not documented in the api for hook_menu in system.api.php. So if you are implementing hook_menu there is not an obvious way to tell this.

I think this should be documented in api.php file.