Minor remarkup updates

Summary:

• Update documentation for changes in D1148.
• Link to Remarkup documentation from Maniphest.
• Support "Note:" syntax in Phabricator (previously, it was only supported in

Diviner, but I've found it pretty good and useful).

Test Plan: Regenerated and perused documentation; made a "NOTE:".

Reviewers: btrahan, broofa, fugalh, jungejason, nh, aran

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: 1149