#4820 Improve docs/coding for drumPitchNames and midiDrumPitches

[Thomas Morley]

Right now there is no documentation how to create custom drum-pitch-names for layout and midi.

In a recent discussion on -user
http://lilypond.1069038.n5.nabble.com/How-can-I-create-from-my-custom-drum-note-to-a-pitch-for-fransposing-td189157.html#a189170
David K wrote:
"There is user documentation and programmer documentation. We are
missing either here. At the moment, these variables have to be
considered internal constants though exported to user space: were they
considered user-accessible parameters, they would need to be defined
using define-session-public.

So there are several things to be done to make them properly
user-accessible."

For now I've created
"Customize drumPitchNames, drumStyleTable and drumPitchTable in layout and midi"
http://lsr.di.unimi.it/LSR/Item?id=1033
with a doc-tag

As soon as this issue is fixed the tag and maybe the entire snippet may be removed.