Add information about admin.

app/views/help/permissions.html.haml

@@ -2,6 +2,7 @@
   %h3.page-title Permissions
   %p.light Users have different abilities depending on the access level they have in particular group or project.
   %p.light If a user is both in a project group and in the project itself the highest permission level is used.
+  %p.light If a user is a GitLab administrator they receive all permissions.
   %hr
 
   %h4 Project: