Doc: Issue 4059: Document MIDI mapping and MIDI effects (issue 249980043 by address@hidden)

[k-ohara5a5a]

Sorry I'm late.
You might still slip in a correction, or just remove mention of
midiChannelMapping='voice since we haven't found a use for it.


https://codereview.appspot.com/249980043/diff/1/Documentation/notation/input.itely
File Documentation/notation/input.itely (right):

https://codereview.appspot.com/249980043/diff/1/Documentation/notation/input.itely#newcode3279
Documentation/notation/input.itely:3279: @ref{MIDI channel mapping}).
This is the part that doesn't work.  We could say
"the MIDI channel associated with the current Staff (or the current the
Voice if you have moved the Staff_performer to the Voice context).  If
you set midiChannelMapping to 'instrument, all staves with the same
instrument are affected when you set one of these context properties.
Setting midiChannelMapping to 'voice often prevents the context
properties form having effect."

https://codereview.appspot.com/249980043/