[luatex] callback documentation

[David Carlisle]

hi,

The new build_page_insert callback is missing its function signature I
think it's

\startfunctioncall
function(<string> extrainfo)
        return <number> skip_register
end
\stopfunctioncall

Also in the previous section, the description of contribute_filter
seems a little bit confusing, it is split between [8.4.1
contribute_filter] and [8.4.2 buildpage_filter and contribute_filter]
with apparently different tables of the allowed values of the
extrainfo/group_codes string in each case?


David