Rollup merge of #81592 - GuillaumeGomez:rustdoc-ui-fixes, r=Nemo157

Rustdoc UI fixes

The first commit fixes this bug (I couldn't figure out why we were setting the width manually and it works as expected without so...):

![Screenshot from 2021-01-31 12-58-46](https://user-images.githubusercontent.com/3050060/106384371-d56a7700-63ca-11eb-9e04-c06b40c2ab5e.png)

The second commit fixes a small bug. On tablets or computer with very little width, the search section goes "over" the search input, making it impossible to click on the search input:

![Screenshot from 2021-01-31 13-22-37](https://user-images.githubusercontent.com/3050060/106384413-021e8e80-63cb-11eb-8321-391a1f8a4c7e.png)

The third and last commit fixes two bugs that you can see in this screenshot:

![Screenshot from 2021-01-31 13-41-05](https://user-images.githubusercontent.com/3050060/106384424-0cd92380-63cb-11eb-82de-76218286c3fb.png)

The wheel is going over the search input and the search tab is going under the search results text. The bug was fixed by simply switching to "mobile mode" at a bigger width:

![Screenshot from 2021-01-31 13-49-50](https://user-images.githubusercontent.com/3050060/106384466-4447d000-63cb-11eb-9330-a7cd29403905.png)

cc ```@pickfire```
r? ```@Nemo157```

0 files changed, 0 insertions, 0 deletions