ZF-8958: Add a description for Zend_Filter_Alpha

Description

Add a description for Zend_Filter_Alpha

Comments

[SVN:r22240:wilmoore] added manual chapter/section with description and examples for Zend_Filter_Alpha.

Reopened issue

This was a notification for myself. The added section is a duplicate from Zend_Validate (which I wrote). Needs to be validated and rewritten.

modificaions to chapter for Zend_Filter_Alpha and also got rid of a problem still being reported by the doc checker in Zend_Filter_BaseName doc.