Add the ability for CMDocument to document roles.

Problem/Motivation

As a site architect it is important to document the intent of a role. Indications for use, and indications for not granting the role.
As such, role entities ought to be open to associate with a CMDocument.

Steps to reproduce

Proposed resolution

Remaining tasks

• Add role entity as a choice on setting form.
• Add roles to docuemented_entity selector on document add/edit form.
• Add alias handling so the alias becomes /admin/people/roles/manage/ROLEmachineNAME/document
• Add tab handling so that when you look at a role /admin/people/roles/manage/ROLEmachineNAME you see a tab for Documentation

User interface changes

API changes

Data model changes