Properly document hotkeys in generated documentation.

Continuing from #15012, this makes four changes to the `rustdoc` static files.

- Change the placeholder text of the search bar to `Click or press 'S' to search, '?' for more options...` to make keyboard hotkeys more apparent (capitalizing the `S` to match the help text).
- Change the `main.js` file to use browser-normalized key codes (`e.which`, from `jQuery`), instead of `e.keyCode`.
- Change the key code for `?` to be the correct `191` instead of `188`, so that the hotkey works to bring up search information.
- Change the search information to display `tab` and `shift+tab` instead of `up` and `down`, as those do not yet work outside of Firefox (see #15011). Also, adjust the height so it does not cut off the help text.


I've also opened up #15038 about the non-functional `up` and `down` functionality, although this does nothing to fix it.

It was changed in a rebase. Should it just be changed back?