R1:9c1697383c38

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…