Doc: Added \compoundMeter function to NR (issue4837050)

[pkx166h]

Reviewers: ,

Message:
Patch ready for review.

But also see

http://code.google.com/p/lilypond/issues/detail?id=1776#c6

as there are still some points I am not clear on on this function that
may need more doc additions.

Description:
Doc: Added \compoundMeter function to NR

Tracker 1776.

Added function to Polymetric Notation section removed old
snippets (which have been superceded by this function).

Also took the opportunity to make some minor grammatical changes
to make (hopefully) the other examples and their explanations
clearer.

Some style changes needed (used @subheading instead of @strong{})
Also removed other references to the snippet in the NR
and moved cindex entries to the new location.

Please review this at http://codereview.appspot.com/4837050/

Affected files:
  M Documentation/notation/rhythms.itely
  D Documentation/snippets/compound-time-signatures.ly
  D Documentation/snippets/new/compound-time-signatures.ly