Improve the help text of xrechnung_routeid.

party.py

@@ -14,8 +14,10 @@ class Party(metaclass=PoolMeta):
 
     xrechnung_routeid = fields.Boolean(
         string='X-Rechnung Route-ID',
-        help='Enables the need for an XRechnung route ID at the party ' +
-        'for exporting the XRechnung.')
+        help='When activated an XRechnung route ID must be used '
+        'for this party for X-Rechnung exports.\n'
+        'The route ID must be defined as identifier of type '
+        '"X-Rechnung Route-ID".')
 
     def get_xrechnung_route_id(self):
         """ search for route-id at party, fire-exception if missing