Doc: warning about tracks and MIDI files

[Federico Bruni]

I've read this message:
http://lists.gnu.org/archive/html/lilypond-user/2013-08/msg00148.html

and I've realized something I've missed since I started using LilyPond 4
years ago: LilyPond creates a MIDI track for each staff in a score, which
means that a typical score configuration for tablature (Staff + TabStaff)
should be set to remove the Staff_performer in TabStaff:

\new TabStaff = "Guitar tabs" \with {
  \remove "Staff_performer"
}

If you don't remove it, the track will be doubled, so the file size will be
double as well as the music (which may results in unattended consequences,
like having acoustic and nylon guitar playing at the same time)

Can you add a warning in the Notation Reference?
I would put it in the introduction to MIDI: NR 3.5.1

Thanks
Federico