The thinking here is that having both a Discussion page and comments for
the same page is redundant, and certianly not what you want if you enable
comments for a page. At first I considered making configurable via pagespec
what pages got discussion links. But that would mean testing a new pagespec
for every page, and a redundant config setting to keep in sync. So intead,
take a lead from my previous change to make inlined pages have a comments
link, and change the discussion link at the top of regular pages to link to
their comments.
(Implementation is a bit optimised to avoid redundant pagespec checking.)