Add \hideNote which is like \hideNotes but just \once. (issue 5342047)

[dak]

In general, you should not touch the translations.  We have translators
for that purpose, and they keep track of the original commits
corresponding to some change.

The only exception is when there is a non-compatible syntax change.  In
that case, appropriate conversion rules should be added to
python/convertrules.py, and the changes that running
scripts/auxiliar/update-with-convert-ly.sh do should bring the
documentation tree into compilable state again, including translations.

This does not mark the translations as corresponding to a different
source commit, as the human translation is still missing.  But that's
only your task if you decide to act as translator, and then you need to
do a complete translation, and do it in the translation branch and
update the upstream commit reference.

http://codereview.appspot.com/5342047/