Rollup merge of #90156 - jsha:less-border-bottom-2, r=GuillaumeGomez

Rollup merge of #90156 - jsha:less-border-bottom-2, r=GuillaumeGomez

Remove underlines from non-top docblocks.

We still had a number of places where underlined section headings would
show up, like under Implementations.

Follow-up to #89506 (thanks `@yaymukund!)` and #90036. Related to #59829.

r? `@camelid`

Demo:

[Before](https://doc.rust-lang.org/nightly/std/string/struct.String.html#trait-implementations):

[![image](https://user-images.githubusercontent.com/220205/138402555-b0c0a3ea-ff50-4aad-bb74-6f9e57323807.png)](https://jacob.hoffman-andrews.com/rust/less-border-bottom-2/std/string/struct.String.html#trait-implementations)

[After](https://jacob.hoffman-andrews.com/rust/less-border-bottom-2/std/string/struct.String.html#trait-implementations):

[![image](https://user-images.githubusercontent.com/220205/138402669-d0835bd9-8813-4f0c-8697-f86e9759acec.png)](https://jacob.hoffman-andrews.com/rust/less-border-bottom-2/std/string/struct.String.html#trait-implementations)