Improve remarkup documentation

Summary:
Some user feedback:

• Named link information not present in quick reference.
• Named link information buried in Phriction docs.
• Optional omission of trailing "=" in headers not documented.

Fix these things.

Test Plan: generated, read documentation

Reviewers: btrahan, paularmstrong, Josereyes

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T884

Differential Revision: https://secure.phabricator.com/D1637