Auto merge of #119283 - GuillaumeGomez:warning-block-pos, r=notriddle

Fix display of warning block if it is first element of the top doc block

It fixes the display of the warning block "i" element in case it is the first element:

![Screenshot from 2023-12-23 11-15-48](https://github.com/rust-lang/rust/assets/3050060/99b6796e-2a09-4053-813e-84288ce76c4c)

It now looks like this:

![image](https://github.com/rust-lang/rust/assets/3050060/306b4cf1-3a7d-4681-b0cf-3e721186bfe8)

The update for the `browser-ui-test` framework is because it didn't detect correctly pseudo elements if they ended with a digit or a dash.

r? `@notriddle`

Author: bors