Bug 1662999: Adding missing help file for wall post

Missing from commit 3383d816



behatnotneeded

Change-Id: Ic036972c4fbfbdbf533fa50ac268e3cc96825b31
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>

htdocs/blocktype/wall/lang/en.utf8/help/forms/activityprefs.activity_wallpost_blocktype_wall.html

+<!-- @license http://www.gnu.org/copyleft/gpl.html GNU GPL version 3 or later -->
+<!-- @copyright For copyright information on Mahara, please see the README file distributed with this software. -->
+<h3>Wall post</h3>
+<p>Notification of activity by other users on the wall that you can put on your profile page.</p>
\ No newline at end of file