Avoid confusing log messages when replication completes
Avoid confusing log messages when replication completes

When a replication finishes, the replicator manager (database)
will call couch_replicator:cancel_replication/1 with the purpose
of ensuring the child spec is removed from the supervisor.
This function however is used as well when users explicitly ask
to cancel a replication, and logs messages which don't make sense
when invoked by the replication manager (added by COUCHDB-1271).
This change also adds a comment about this behaviour.

alanning/couchdb-couch,alanning/couchdb-couch,ngpestelos/couchdb-couch,apache/couchdb-couch,smithsz/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,ngpestelos/couchdb-couch,smithsz/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,ngpestelos/couchdb-couch,apache/couchdb-couch,fkaempfer/couchdb,apache/couchdb-couch,KlausTrainer/couchdb-couch,fkaempfer/couchdb,smithsz/couchdb-couch,cloudant/couchdb-couch,KlausTrainer/couchdb-couch,alanning/couchdb-couch,cloudant/couchdb-couch,KlausTrainer/couchdb-couch,fkaempfer/couchdb