Allow more section levels for now
Allow more section levels for now

The new parser can be asked not to allow title-level sections outside
.mld files, since titles are generated by odoc on pages generated from
.mli files. However, the machinery for driving this choice outside the
parser is not developed, so the parser is currently rejecting titles
even in .mld files.

As a temporary workaround, this commit makes the parser always accept up
to one title per comment.

ocaml-doc/odoc,ocaml-doc/odoc