Add a formatting reference link to the inline comment editor

Summary:
Various text boxes have a documentation link below them.
Make the Differential inline comment box one of them.

Test Plan:
Loaded Differential revision in sandbox. Played around
with inline comments.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1513

Differential Revision: https://secure.phabricator.com/D3024