PATCH - DOC: Added @knownissue to NR for fingering

[James Lowe]

http://codereview.appspot.com/4248081/

Doc: Added @knownissue to NR for fingering

By default double-digit fingering (i.e. >9) is not possible using
normal fingering markups. There is an LSR that has been posted for approval
at the time of this patch, and will be included in the snippet link in the
@seealso already in this section.

Tidied up some grammar and minor punctuation changes.

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Came out of

http://article.gmane.org/gmane.comp.gnu.lilypond.general/62604

James

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