* Split out the machinery which parses and sets the heading to make a
new environment `describe*' which can have multiple headings.
* Remove the `tabular' layer in the headings, by using `\rlap'
instead.
* Fix spacing around the header lines.
* Insert penalties and drive the `\if@nobreak' and `\@afterheading'
machinery correctly so that we mostly don't get bad page breaks.