admin/groups: Update group short name info

Only lowercase short names are accepted.

source/administration/groups.rst

@@ -273,7 +273,11 @@ Here is a full list of all the possible fields that you can specify:
    * course: member, tutor, administrator
    * standard: member, administrator
 
-* **shortname**: This is the name used to identify this group in the database. This should be only one word. It is also used to identify the group when you :ref:`add or update its members by CSV <groups_update_members>`.
+* **shortname**: This is the name used to identify this group in the database. It is also used to identify the group when you :ref:`add or update its members by CSV <groups_update_members>`.
+
+  .. note::
+    The group shortname needs to be lowercase.
+
 * **submitpages**: Choose to allow the submission of pages and collections to this group. If you create a course group, this setting is automatically chosen.
 * **suggestfriends**: Allow group members to send their friends a recommendation to join this group.
 * **usersautoadded**: Choose to have all new users on the site added to the group automatically.