Doc: NR Warning added to para for cueduring (issue4850051)

[tdanielsmusic]

LGTM, with a tweak as suggested below to explain why.


http://codereview.appspot.com/4850051/diff/1/Documentation/notation/staff.itely
File Documentation/notation/staff.itely (right):

http://codereview.appspot.com/4850051/diff/1/Documentation/notation/staff.itely#newcode1304
Documentation/notation/staff.itely:1304: @warning{In this example, the
@code{Voice} context must be explicitly
@warning{When a @code{Voice} starts with @code{\cueDuring}, as in the
following example, ... }

http://codereview.appspot.com/4850051/