I noticed just now that commenting blocks with --[ and ]-- works, despite the fact that the highlighting does not show it.
Yes. It is part of lua 
its helpful.
I guess you mean ‘the fact that it comments’ and not ‘the fact that it doesn’t show as commented in Codea’? 
@Herwig… a request for the correct highlighting of ‘block comments’ was added to the wiki some time ago… including this special form (for cases where your code has nested square brackets):
–[=[…]=]
https://bitbucket.org/TwoLivesLeft/codea/issue/55/text-editor-does-not-recognise-block
Sorry about not having this feature yet. It’s a tricky one to implement given the way the syntax highlighting works.