Doc: Added \cueClef command (issue4601060)

[percival . music . ca]

LGTM


http://codereview.appspot.com/4601060/diff/1/Documentation/notation/staff.itely
File Documentation/notation/staff.itely (right):

http://codereview.appspot.com/4601060/diff/1/Documentation/notation/staff.itely#newcode1221
Documentation/notation/staff.itely:1221: \new CueVoice
could these be
  \new CueVoice {
    \stemUp d'8^"flute" c d e fis2
  }
or
  \new CueVoice { \stemUp d'8^"flute" c d e fis2 }
instead?  that's more in keeping with the doc .ly style elsewhere.

Yes, we'll have a GOP discussion about .ly formatting, but for now if
you do the same stuff as the rest of the docs, there's no reason to hold
up this patch.

http://codereview.appspot.com/4601060/