Staff.instrument vs. .midiInstrument

[Graham Percival]

(for those with some formal logic training.  I'm not certain if the joke
will translate, though)
Is this a Well-Formed File?

----
\score {
   \context Staff=sop {
      \property Staff.instrument="soprano"
      \notes{ a4 a b c'}
   }
   \paper{}
   \midi{}
}
----

It produces a warning that it can't find instrument "soprano" (this
is Ricardo Kirkner's question that appeared on -user a few days
ago).  Since he didn't define Staff.midiInstrument, LilyPond took
the value for Staff.instrument and naturally couldn't find a
midi soprano instrument.

Should I update the docs to warn users that if they use Staff.instrument
they should set Staff.midiInstrument as well (if only to set it to
the default piano sound), or do you want to change the way LilyPond
treats .midiInstrument and .instrument?

Cheers,
- Graham