Auto merge of #148466 - liigo:better-rustdoc, r=GuillaumeGomez

rustdoc: quality of life changes

- Support `=` shortcut (alongside `+`) to expand all sections. Already support `s` and `S`, `Shift` or not.
- ~~Fix search-input's placeholder. The input is auto focused, any key press will be accepted as input not shortcut, current placeholder is missleading.~~

Author: bors