Doc: NR 1.2.6 added slashedGrace function (issue 5133043)

[n . puttock]

http://codereview.appspot.com/5133043/diff/1/Documentation/changes.tely
File Documentation/changes.tely (right):

http://codereview.appspot.com/5133043/diff/1/Documentation/changes.tely#newcode65
Documentation/changes.tely:65: There is a new @code{\slashedGrace}
function:
This is already documented in changes.itely

http://codereview.appspot.com/5133043/diff/1/Documentation/notation/rhythms.itely
File Documentation/notation/rhythms.itely (right):

http://codereview.appspot.com/5133043/diff/1/Documentation/notation/rhythms.itely#newcode3051
Documentation/notation/rhythms.itely:3051: grace notes with a slashed
stem using the @code{\slashedGrace} function.
Need to explain why.  \slashedGrace doesn't have a slur, so its main use
would be to show a note sounded early via a tie.

http://codereview.appspot.com/5133043/