rustdoc: The default style of items inside "Implementations on Foreign Types" is very ugly

[kennytm]

Demo: https://doc.rust-lang.org/beta/alloc/slice/trait.SliceIndex.html

There are many unnecessary elements applied to the "Implementations on Foreign Types" which probably should all be removed.

cc @QuietMisdreavus (#43849)

[GuillaumeGomez]

Wo, the style is completely broken. I'll fix it.