help-engraving-mode: comments about P-a-C info

Committing an extremely old modification ...
Merge pull request #427 from uliska/engraving-help

help-engraving-mode: comments about P-a-C info
Avoid python crash mentioned in issue #331
Merge pull request #430 from PeterBjuhr/musicxml

Avoid python crash mentioned in issue #331
pot; some nl updates