duplicated material in docs (music functions)

[Mark Polesky]

There's duplicated material in these two sections:
  NR 5.6 Using music functions
  EX 2.1 Music functions

I'd rather
* reduce NR 5.6 to a tiny stub with a cross-reference to
  EX 2.1
* reword EX 2.1 to match the more newbie-friendly tone of
  NR 5.6

Any objections to this idea?

By the way, what's the proper abbreviation for "Extending"?
It's not mentioned in CG 4.3.2 "Documentation files".

- Mark