'artifact log': document the 'artifacts' argument

I found it quite difficult to guess what format the bst artifact log command would like artifact references in. Put them in the help for that command, to save folks time.

In future work, we probably want to document the behaviour of 'Stream._classify_artifacts' globally - most commands take them and the results can be surprising if you don't know about artifact refs.

Helps with #959 (closed)