Use tags/links to show extended classes in Diviner, and fix a minor empty state thing

Summary: Ref T988. Show "Extends:" as linked tags. Fix the style of "This <top-level thing, like a class or function>" is not documented so it's the same as "This method is not documented.".

Test Plan:
Tags thing before:

{F57557}

Tags thing after:

{F57558}

Undoc before:

{F57559}

Undoc after:

{F57560}

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D6910