Add html <details> element to allowed comment syntax

I think it would be nice to be able to throw some extra detail in comments from time to time that shouldn't be generally shown without clicking expand. Things which aren't core to the discussion but help support the comment are worth doing here. Opening in meta because this feels less like the sort of thing that belongs in a github issue and more like something that would be useful for users of the site to discuss / have opinions on. I.e.: some text The rest of the long post