[mb-users] [unknown] is perhaps not very unknown, and [unknown] != [unknown]

Just mentioning how I've been handling a case that I guess comes up
now and then, in case anyone has comments.

=== [unknown] is not very unknown ===

Say there are two pianists on a release. On a particular track on that
release there is a piano, but you don't know which one is playing.
I have sometimes added ARs for the pianists to the release and then an
AR for [unknown] playing the piano on that recording.
I see nothing wrong with that, but would like to mention it anyway.

A recent example is
https://musicbrainz.org/release/30036a89-167c-44bd-bd50-12ed5b9c105d
where the credits list what musicians play what on the release, and
also what instruments there are on the tracks,
so that you often don't know which one of two who are playing on a
particular track.

=== [unknown] is not the same as [unknown] ===

For this release there was something new to me. There are two
bassoonists and two oboeists, so when it said about a track that
bassoon and oboe was played I noted it as  "bassoon and oboe:
[unknown] ". If this is taken to indicate that there is an unknown
musician who plays the bassoon *and* the oboe in this recording this
is of course wrong. But I guess it shouldn't be interpreted that
way (and that if we actually had a case like that, that'd be reason to
create an artist for that particular unknown artist).