Rollup merge of #74635 - GuillaumeGomez:fix-tooltip-pos, r=Manishearth
Rollup merge of #74635 - GuillaumeGomez:fix-tooltip-pos, r=Manishearth

Fix tooltip position if the documentation starts with a code block

Fixes #74321.

Before:

![before](https://user-images.githubusercontent.com/3050060/88188970-cf842400-cc38-11ea-839b-37e41656837d.png)

After:

![after](https://user-images.githubusercontent.com/3050060/88188981-d3b04180-cc38-11ea-8194-713ffe640d3a.png)

And in case there is text, it is not being applied:

![after-witness](https://user-images.githubusercontent.com/3050060/88189009-ddd24000-cc38-11ea-9f0a-61dfd0a0cbd0.png)

And on mobile it isn't needed so it's not "activated":

![Screenshot from 2020-07-22 17-17-43](https://user-images.githubusercontent.com/3050060/88194698-65bb4880-cc3f-11ea-8513-0043ccca8cfc.png)

r? @rust-lang/rustdoc

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