Issue 3958: Give examples of \applyContext usage.

The documentation on \applyContext is pretty slim.  I'm just mentioning some new tricks I recently learned, in the hopes that it will benefit future users/developers.

http://code.google.com/p/lilypond/issues/detail?id=3958

[janek, 2014-06-23 09:15:55]

LGTM - this is helpful, thanks!

[Keith, 2014-06-24 06:03:49]

Looks fine as-is.
You might look at it again, and see if you like the idea of shortening it so
more people read it.

https://codereview.appspot.com/110060045/diff/1/Documentation/extending/progr...
File Documentation/extending/programming-interface.itely (right):

https://codereview.appspot.com/110060045/diff/1/Documentation/extending/progr...
Documentation/extending/programming-interface.itely:1170: The example may seem
artificial, but as will be described later,
Maybe skip to the non-artificial example.

https://codereview.appspot.com/110060045/diff/1/Documentation/extending/progr...
Documentation/extending/programming-interface.itely:1233: The function passed to
@code{\applyContext} can set properties for
This example alone seems to make the important point.  I think people could
easily generalize from this example to what you show in the others, or to
whatever else they want to do.

[dak, 2014-06-24 19:35:18]

https://codereview.appspot.com/110060045/diff/1/Documentation/extending/progr...
File Documentation/extending/programming-interface.itely (right):

https://codereview.appspot.com/110060045/diff/1/Documentation/extending/progr...
Documentation/extending/programming-interface.itely:1339: @end lilypond
This passage is too long.  One thing that is worth mentioning is that
\applyContext tends to be a last resort.  When one quotes music or otherwise
manipulates or replays it, the actions taken by applyContext are basically
opaque.  \set/\override/\revert are turned into corresponding stream events and
one can simulate or interpret the actions they are taking and knows which
properties are affected.

\applyContext, in contrast, has arbitrary effects.  While it also can be
recorded and replayed as a stream event, there is no way of knowing its effects
on the context's properties without actually making the call and comparing the
context properties before and afterwards.  And its effects will depend on the
status of the current context, to boot.

[Mark Polesky, 2014-07-04 10:54:35]

I've rewritten the \applyContext description, and this new patch is about 40%
shorter than the previous one.  Please review.

Thanks,
Mark

[janek, 2014-07-04 21:07:24]

LGTM, thanks!