Doc: NR Section on Upbeats made clearer

Doc: NR Section on Upbeats made clearer

Tracker issue 2189

Created simpler and clearer @lilypond examples.

Used a less complex, more obvious value for \set Timing.measure position
examples and added a specific example after the @knownissue to show
\partial and \set Timing.. being used together (at the beginning and
after the beginning) to help reinforce the point.

Minor cindex changes and linebreak edits as per the CG