We recently adopted a new standard way to document "callback functions" -- see http://drupal.org/coding-standards/docs#callback-def for the standard and #1250500: [policy adopted; patch done] Find a (standard) way to document callback signatures for the issue where this was adopted.
The FormAPI #element_validate callback should be documented as part of this.
This would also require the form API docs at http://api.drupal.org/api/drupal/developer%21topics%21forms_api_referenc... to be changed to refer to the new callback documentation.