Auto merge of #87818 - GuillaumeGomez:anchors-display-rustdoc, r=camelid
Auto merge of #87818 - GuillaumeGomez:anchors-display-rustdoc, r=camelid

Fix anchors display in rustdoc

Fixes https://github.com/rust-lang/rust/issues/87611 (it simplifies the positioning and fix the background).

![Screenshot from 2021-08-06 16-47-03](https://user-images.githubusercontent.com/3050060/128531105-61d1c21f-4a4d-4d68-aedf-9bfe0332f8ae.png)
![Screenshot from 2021-08-06 16-47-10](https://user-images.githubusercontent.com/3050060/128531109-b2ea8065-10b0-4400-9507-322122e42e78.png)
![Screenshot from 2021-08-06 16-47-14](https://user-images.githubusercontent.com/3050060/128531111-8a17cbdb-29e8-4baa-a0d6-81aa4f6ac6ed.png)

r? `@camelid`

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