[2877] Description for log messages

Provide descriptions for log messages added in the previous commit, the
ones for more detailed logging during update of database based
datasource.

src/lib/datasrc/datasrc_messages.mes

@@ -567,13 +567,27 @@ This indicates a programming error. An internal task of unknown type was
 generated.
 
 % DATASRC_DATABASE_ADDDIFF updated diff table for add: %1 %2 %3
+Debug message. A difference record for adding a record to the zone is being
+appended to the difference table. The name, type and rdata of the record is
+logged.
 
 % DATASRC_DATABASE_DELETEDIFF updated diff table for delete: %1 %2 %3
+Debug message. A difference record for removing a record from the zone is being
+appended to the difference table. The name, type and rdata of the record is
+logged.
 
 % DATASRC_DATABASE_ADDNSEC3 added NSEC3 RR: %1 %2
+Debug message. A new NSEC3 record is added to the table. The hash and the rdata
+is logged.
 
 % DATASRC_DATABASE_ADDRR added RR: %1 %2 %3
+Debug message. A new resource record is added to the table. The name, type and
+rdata is logged.
 
 % DATASRC_DATABASE_DELETENSEC3 deleted NSEC3 RR: %1 %2
+Debug message. An NSEC3 record is removed from the table. The name, type and
+rdata is logged.
 
 % DATASRC_DATABASE_DELETERR deleted RR: %1 %2 %3
+Debug message. A resource record is removed from the table. The name, type and
+rdata is logged.