Doc addition

[Cameron Horsburgh]

Here's a small addition for the docs. It is intended to go in Section
8.2.3 (Rehearsal marks) after the sentence "You may use
format-mark-barnumbers, format-mark-box-barnumbers, and
format-mark-circle-barnumbers to get bar numbers instead of
incremented numbers or letters."

Given the overlap of subject material it may also be an worthwhile
putting a link to Section 8.1.3 (Text marks) in the 'See also'
paragraph.

******************
Other styles of rehearsal mark can be specified manually

\mark "A1"

Score.markFormatter does not affect marks specified in this manner.
However, it is possible to apply a \markup to the markup string

\mark \markup{ \box A1 }
******************

Have a nice day!
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Cameron Horsburgh

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