Doc: NR 5.3 Add \single command (issue 6742057)

[pkx166h]

Reviewers: ,

Message:
Here is the first draft of reworking NR 5.3 while adding the \single
command.

I _expect_ to have lots of edits and go through a few iterations of
this, the point is to get something moving in terms of updating this
section, especially with recent discussions about the new commands
(\single, \omit, \no etc.) where it was evident that some were unhappy
about it.

I don't claim to understand many of the finer points so I have not added
any new information (apart from the \single command) but have rearranged
this section in a more logical order and removed some (I think)
unnecessary cruft, repetition, syntactically poor grammar. Of course
this is all my own opinion, but I saw no point in just tacking yet
another subsubheading on to the end of this section just to tick the 'I
have documented \single' box.

The @lilypond examples are as they were before as I'd like to at least
get the technical descriptions right before I attempt to put some better
@lilypond examples - for example make some less complex and include more
for commands like \single and \tweak especially.

Description:
Doc: NR 5.3 Add \single command

Add the new \single command.

This section has also had a tidy up to rearrange some parts to avoid
repetition, simplify the explanations, remove some redundant
explanations,
give more/better @lilypond examples.

Please review this at http://codereview.appspot.com/6742057/

Affected files:
  M Documentation/learning/fundamental.itely
  M Documentation/notation/changing-defaults.itely
  M Documentation/notation/editorial.itely
  M Documentation/notation/rhythms.itely