rustdoc: list elements containing multiple paragraphs have confusing margins

[lolbinarycat]

take the feature flags on the time crate.

the descriptions are grouped closer to the next element, instead of the one they actually apply to!

here's my simple fix:

.docblock li {
  margin-bottom: .8em;
}

.docblock li p {
  margin-bottom: .1em;
}

here's what it looks like with this fix applied:

[lolbinarycat]

waiting to open a PR until #129545 is merged.

[sloanev]

Note for everyone's convenience: this is a duplicate issue of #130622.