Bring appendix sectioning up to date

Among OTS's docs that used this unnumbered/numbered sectioning system,
there were two variants (well, really three, but one of the variations
was just the presence or absence of a comment about appdxsection being
"related to CAMH template stuff that is too boring to go into here";
that comment seems likely to be obsolete so I'm not porting it over).
The newer of the two variants was only used in one doc, but that doc
was the most recent one, so what it has is probably what we want:

* Add comment about using "\setcounter{section}{0}"

* Include a colon, so it's "Appendix <letter>:"