Doc: add doc-strings to music-functions

Doc: add doc-strings to music-functions

The \harmonicsO[n|ff] commands have been
rewritten as music-functions, to be more
like other definitions such as \palmMute.

This commit also adds a \harmonicNote command,
for consistency's sake with \deadNote etc.,
and a \defaultNoteHeads that acts as a common
definition for all note head styles reverting
(i.e. \deadNoteOff, \harmonicsOff etc.).