Add help file for remoteusername (bug #810302)

Change-Id: I4e9ba99028f5fed2702c71f7327624ba72e30399
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>

htdocs/admin/users/edit.php

@@ -167,6 +167,7 @@ if (count($authinstances) > 1) {
                 'title'        => get_string('remoteusername', 'admin'),
                 'description'  => get_string('remoteusernamedescription', 'admin', hsc(get_config('sitename'))),
                 'defaultvalue' => $un ? $un : $user->username,
+                'help'         => true,
             );
         }
     }

htdocs/lang/en.utf8/help/forms/edituser_site.remoteusername.html

+<h3>Username for external authentication</h3>
+<p>When a user is authenticated by some service external to Mahara,
+this field contains the identifier for the user on the remote service.
+It is possible for a user to have several remote usernames stored in
+Mahara, but each one must be associated with a different external
+authentication method.</p>
+<p>The current rules for the update of this field are as follows:</p>
+<ul>
+  <li>If you wipe the contents of this field and submit, any external
+  username associated with this user and the selected authentication
+  method will be deleted.</li>
+  <li>If you update the contents of this field to anything other than
+  the default, the external username will be reset, or a new one will
+  be created if it doesn't exist yet.</li>
+  <li>If you leave the contents of this field on the default and
+  submit, a new external username will be created if one doesn't exist
+  yet.  However, if the user already has an external username for the
+  selected authentication method, and it's different to the default,
+  it will not be overwritten.</li>
+</ul>