Enhancement request/partial patch: Document the MIDI backend

[Johannes Rohrer]

Dear LilyPond developers,

a month ago, on a tangent to the Issue #687 discussion 
<http://code.google.com/p/lilypond/issues/detail?id=687>, I got tempted 
to have a look at the LilyPond MIDI backend, only to be scared away by 
the utter lack of documentation.

However, I have looked again a few times since then. Now I would like to 
file a wish to improve that situation, and offer some help.

At least, I would like to see the following enhancements:

1. Improve performer documentation in the Internals Reference to 
engraver standards:

   a) document performer/context relations as specified
      in ly/performer-init.ly

   b) document which music types are accepted by performers

   c) document Audio Items (analogously to grobs) produced by performers

2. Provide at least a short MIDI backend overview in the Contributor's 
Guide.


Here is a patch that implements 1 a) and b).

<https://codereview.appspot.com/6868047>

Should you accept it, I am ready to tackle 1 c).

Best regards,

Johannes Rohrer