Insert instructions in admin page and permissions document

app/views/admin/application_settings/_form.html.haml

@@ -26,7 +26,9 @@
         .btn-group{ data: data_attrs }
           - restricted_level_checkboxes('restricted-visibility-help').each do |level|
             = level
-        %span.help-block#restricted-visibility-help Selected levels cannot be used by non-admin users for projects or snippets
+        %span.help-block#restricted-visibility-help
+          Selected levels cannot be used by non-admin users for projects or snippets.
+          If public level is restricted user profiles are not accessible to not logged users.
     .form-group
       = f.label :import_sources, class: 'control-label col-sm-2'
       .col-sm-10

doc/permissions/permissions.md

@@ -93,3 +93,10 @@ An administrator can flag a user as external [through the API](../api/users.md)
 or by checking the checkbox on the admin panel. As an administrator, navigate
 to **Admin > Users** to create a new user or edit an existing one. There, you
 will find the option to flag the user as external.
+
+## Restricted visibility levels
+
+Visibility levels can be restricted in admin settings page by administrator, when
+restricting a visibility level groups, projects and snippets are not allowed to be
+created with that visibility setting. If the public visibility level is restricted
+user profiles are accessible to not logged users.