ZF-8961: Add a description for Zend_Filter_Dir

Description

Add a description for Zend_Filter_Dir

Comments

added manual chapter/section with description and examples for Zend_Filter_Dir. Also corrected a typo in Zend_Filter_Digits. Implemented via [SVN:r22243:wilmoore].

Reopened issue

This was a notification for myself and the added section is incomplete.

modificaions to chapter for Zend_Filter_Dir per documentation spec. Documentation test is passing.