The Markdown options processor cannot handle rendering tables
to DocBook. This could be fixed, but as we won't be using the
DocBook output for long I just removed them for now in the interest
of expediency; they were all well-suited to being description lists
showing option types anyway, apart from one awkward case in the form
of trayer, which also had ad-hoc syntax for enumerating acceptable
values in the documentation. Since the types aren't actually used for
option processing anyway, I changed them to use `enum` and similar to
give a single description of the acceptable values without a big table.
