Rollup merge of #90036 - jsha:less-rule, r=GuillaumeGomez

Rollup merge of #90036 - jsha:less-rule, r=GuillaumeGomez

Remove border-bottom from most docblocks.

Headings in the top-doc docblock still get a border-bottom due to a rule
that covers all h2, h3, and h4. Method docblocks are generally h5, and
so don't get a border-bottom anymore.

This fixes a problem where a sub-sub-heading within a method would have
a line that went all the way across the page, creating a division that
made that sub-sub-heading look much more important than it really is.

Fixes #90033

Demo at https://jacob.hoffman-andrews.com/rust/less-rule/std/string/struct.String.html

r? ``@GuillaumeGomez``