quick addition before 2.11.35

[Graham Percival]

Hi John (or anybody),

Could you merge this fix before 2.11.35 ?  It really helps the Learning 
Manual, and would simplify our jobs if these predefined commands made it 
into the .35 binary.

http://git.sv.gnu.org/gitweb/?p=lilypond.git;a=commitdiff;h=8ddd9b224b19bdd45d7e160d357b8a8dc1eb6e24


I know that we don't want to go overboard with the predefined commands, 
but these _really_ do help the docs... and they're no worse than the 
\improvisationOn commands.  I normally wouldn't ask for something like 
this, but I really think it's worth it in this case.

Cheers,
- Graham