instruments: get rid of the "group-name" argument

I think that it was overkill; i don't want to go into that much detail when
defining new instruments.  Besides, it seems that writing just

\context {
  \StaffGroup
  \accepts #staff-name
}

will make the instrument \accepted by Score, StaffGroup and ChoirStaff.
That should be enough for us.
instruments: reorder args of \newInstrument and make some optional.

This is done according to the suggestion by David Kastrup (thanks!).
instruments: cleanup description