Custom percussion staves section.
If you do not like any of the predefined lists you can define your own list at the top of your file.
I would call them predefined styles instead of predefined lists, as this is how they were presented
above. Then I would explain that these styles, internally, are Scheme lists
which you can define yourself.
split off from https://sourceforge.net/p/testlilyissues/issues/5297/
Doc: NR - percussion.itely - Custom Percussion staves intro
Issue 5406
Improve introductory paragraph.
http://codereview.appspot.com/351780043
Passes make, make check and a full make doc.
Patch on countdown for 1st September
Patch counted down - please push.