Doc: Added \compoundMeter function to NR

This is for Tracker issue 1776

Added function to Polymetric Noation sections removed an old
snippet from NR (which has been deprecated by this function).

Also took the opportunity to make some other 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 snipped including in the
translated tely files and moved cindex entries to the new location.