Rollup merge of #36676 - bluss:rustdoc-where-css, r=steveklabnik
Rollup merge of #36676 - bluss:rustdoc-where-css, r=steveklabnik

rustdoc css: Put `where` in trait listings on a new line

This is about the gray area at the top of a trait's documentation page,
that lists all methods and their signatures. A big trait page like
Iterator is very crowded without this tweak.

aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust