Add a link to Remarkup Reference below comment

Summary:
To reduce blindness, all textareas with some kind of special syntax should have
an information about this syntax and a link to its documentation. Preview
function is a nice complement but it doesn't replace this information.

I've added this information and the link below the comment field.

Please note that <a target> is a valid attribute in HTML5.

Test Plan:
Go to https://secure.phabricator.com/D1164#comment-content
There should be a link to Remarkup Reference
This link should open Remarkup Reference in a new window (to not discard the
comment)

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley, vrana

Differential Revision: 1164