rustdoc: short vertical spacing on associated types

[jsha]

On https://doc.rust-lang.org/nightly/std/ascii/struct.EscapeDefault.html#impl-Iterator-for-EscapeDefault, the spacing below type Item = u8 and below Item's docblock is too short. Nightly:

Stable (correct):

I think this is related to #103364, since the selector that is supposed to give the summary a margin-bottom: 0.75em; changed from .implementors-toggle summary to .implementors-toggle > summary.

/cc @notriddle