introducing input modes in NR

[Graham Percival]

What punctuation do we want to denote different input modes?  ie

A @code{\markup} block is used to typeset text with an extensible
specific syntax called @qq{markup mode}.

Chords can also be entered in @notation{chord mode}...

Lyrics are written in @code{lyric mode}.


IMO, we shouldn't use @code{}, since that's useful to reserve for
exact .ly input.  @notation{} currently is only used for actual
musical terms, and I think it's useful to keep it that way.  @qq{}
seems like the only thing left, but for some reason I don't like
the way it looks.

Any suggestions?  Or simply people telling me "there's nothing
wrong with @qq, get back to work on your thesis!"?

Cheers,
- Graham