Doc: elaborate explanation of accidentals within cadenzas (3078) (issue 8866043)

[ianhulin44]

LGTM apart from a Nitpick:
"can" implies there just may be an option of the accidentals appearing
in this case. So use "must" "have to" or "need to be".
Cheers, Ian


https://codereview.appspot.com/8866043/diff/1/Documentation/notation/rhythms.itely
File Documentation/notation/rhythms.itely (right):

https://codereview.appspot.com/8866043/diff/1/Documentation/notation/rhythms.itely#newcode1514
Documentation/notation/rhythms.itely:1514: reminder accidentals can be
inserted manually, see @ref{Accidentals}.
"must be inserted manually"

https://codereview.appspot.com/8866043/