Document role filtering.

Role filtering doesn't seem to be documented, and was added in a pull
request before v3 was released. This commit adds documentation for the
existing methods of setting the role filter, and how it works, as well
as the command line setting which is a pull request currently open.
Add role filtering page to navigation
Don't use underscore emphasis style
Merge pull request #45 from forward3d/role_filtering

Document role filtering.