[1512] added detailed log description.

src/lib/python/isc/ddns/libddns_messages.mes

@@ -16,10 +16,26 @@
 # of the libddns_messages python module.
 
 % LIBDDNS_UPDATE_ERROR update client %1 for zone %2: %3
-TBD
+Debug message.  An error is found in processing a dynamic update
+request.  This log message is used for general errors that are not
+normally expected to happen.  So, in general, it would mean some
+problem in the client implementation or an interoperability issue
+with this implementation.  The client's address, the zone name and
+class, and description of the error are logged.
 
 % LIBDDNS_UPDATE_FORWARD_FAIL update client %1 for zone %2: update forwarding not supported
-TBD
+Debug message.  An update request is sent to a secondary server.  This
+is not necessarily mean invalid, but this implementation does not yet
+support update forwarding as specified in Section 6 of RFC2136 and it
+will simply return a response with an RCODE of NOTIMP to the client.
+The client's address and the zone name/class are logged.
 
 % LIBDDNS_UPDATE_NOTAUTH update client %1 for zone %2: not authoritative for update zone
-TBD
+Debug message.  An update request for a zone for which the receiving
+server has authority.  In theory this is not an unexpected event, but
+there are client implementations that could send update requests
+carelessly, so it may not necessarily be so uncommon in practice.  If
+possible, you may want to check the implementation or configuration of
+those clients to suppress the requests.  As specified in Section 3.1
+of RFC2136, the receiving server will return a response with an RCODE
+of NOTAUTH.