ZF-2755: Document Zend_Validate constants

Description

Now that we can translate Zend_Validate error messages based on error constants, we need to document the available constants and default messages so that users can know what codes to use when translating.

Comments

Duplicate of ZF-1877

Bookkeeping. Assigning all resolved issues to the people who resolved them. The only unassigned issues should be those that are new and unreviewed.