Add link to message translation doc (#2609)

doc/sources/admin/plugincustom.rst

@@ -152,12 +152,14 @@ Plugins may declare additional session fields to be stored in the :doc:`loginhis
             _language => '__hidden__',
 
             # This field will be displayed on the portal. The column name
-            # can be internationalized
+            # is treated like a message and can be internationalized
             authenticationLevel => "Human friendly column name",
         });
         return 1;
     }
 
+Column names can be translated by :ref:`overriding the corresponding message <intlmessages>`
+
 Example
 -------
 

doc/sources/admin/portalcustom.rst

@@ -212,11 +212,12 @@ lemonldap-ng-cli:
 
    /usr/share/lemonldap-ng/bin/lemonldap-ng-cli -yes 1 set portalSkin 'myskin' portalSkinBackground ''
 
-
 You can find additional documentation on the syntax of template files in the
 `official documentation of the HTML::Template module
 <https://metacpan.org/pod/HTML::Template>`__
 
+.. _intlmessages:
+
 Messages
 ~~~~~~~~