Update documentation for #element_validate for using methods as a callback

Thanks for working to improve Drupal.

This is for the Drupal 7 Form API reference. Since Drupal 7 is past end-of-life this is now a "won't fix".

Work on documentation for form elements for supported versions of Drupal is at 📌 [meta] Move/Create Form Element Documentation Active and 🌱 [Meta] Improve documentation for Render and Form Elements Active .