Explain that mongodump --oplog only works against full dumps

The --oplog option cannot be used when --db is in use as well:

$ mongodump --host raw/jaguar.local:27000 --db load --oplog
2017-08-22T14:33:18.503+0200 Failed: bad option: --oplog mode only supported on full dumps

The documentation on https://docs.mongodb.com/manual/reference/program/mongodump/#options for the --oplog option does not mention this restriction.