For one thing I kinda like the annotated php online manual. This is
basically the normal manual, with every chapter having its own page,
and reader comments at the bottom. Unexpected behaviour is mostly
"documented" in the comments.
I also like the wiki approach, so maybe something in between could be
done: put the lua manual on the wiki, with every chapter being a
separate page. Then users can contribute their own findings, ideas,
or samples to the "official" documentation, and it could be augmented
with stuff from this list, like explanations for certain behaviour
from the developer team. This way all documentation-related stuff
would be in one (probably pretty large) place.