Doc: Update changing context default settings (2322) (issue 6345086)

[tdanielsmusic]

Reviewers: ,

Message:
This is a substantial change.  I took the opportunity to
deal with a TODO in the file - document \with - as
this made for a more sensible organisation of the section.

I'd like reviewers to consider in particular

 - factual accuracy (I was not confident about some parts)
 - quality of the examples (are short separate ones better?)
 - and of course general clarity

Trevor


Description:
Doc: Update changing context default settings (2322)

 - rewrite complete section to include description of
   \with and add possibility of including predefined
   commands in \context blocks in \layout

 - uses shorter, more specific examples

Please review this at http://codereview.appspot.com/6345086/

Affected files:
  M Documentation/notation/changing-defaults.itely