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

src/applications/differential/view/addcomment/DifferentialAddCommentView.php

@@ -104,7 +104,15 @@ final class DifferentialAddCommentView extends AphrontView {
           ->setID('comment-content')
           ->setLabel('Comment')
           ->setEnableDragAndDropFileUploads(true)
-          ->setValue($this->draft))
+          ->setValue($this->draft)
+          ->setCaption(phutil_render_tag(
+            'a',
+            array(
+              'href' => PhabricatorEnv::getDoclink(
+                'article/Remarkup_Reference.html'),
+              'target' => '_blank',
+            ),
+            'Remarkup Reference')))
       ->appendChild(
         id(new AphrontFormSubmitControl())
           ->setValue($is_serious ? 'Submit' : 'Clowncopterize'));

src/applications/differential/view/addcomment/__init__.php

@@ -20,6 +20,7 @@ phutil_require_module('phabricator', 'view/form/control/tokenizer');
 phutil_require_module('phabricator', 'view/form/error');
 phutil_require_module('phabricator', 'view/layout/panel');
 
+phutil_require_module('phutil', 'markup');
 phutil_require_module('phutil', 'utils');