Update documentation for MIDI articulations.

Update documentation for MIDI articulations.

Also clarify what articulate.ly supports.

Change the snippet for redefining a shorthand to use an ArticulationEvent
instead of an articulation name, so users who follow its example will have
their articulations work in MIDI.

[Trevor Daniels, 2013-12-16 11:57:33]

LGTM, apart from a slight rewording.

https://codereview.appspot.com/38420044/diff/1/Documentation/notation/input.i...
File Documentation/notation/input.itely (right):

https://codereview.appspot.com/38420044/diff/1/Documentation/notation/input.i...
Documentation/notation/input.itely:2891: @ref{The Articulate script} adds the
following items to the above list:
While you're changing this sentence, could it be 
reworded to put the @ref at the end?  It looks 
better that way in the pdf.
Like this:
"A script is available which adds the following items
to this list, see @ref{The Articulate Script}:"

Trevor

[Devon Schudy, 2013-12-17 15:34:27]

Move @ref to end of line; mention glissandi.