(doc) Properly doc hotkeys in generated docs.

Updated search bar to match help text.
Used correct, normalized hotkeys in search.
Updated shortcut menu with working shortcuts (tabs).
Changed height of search help.
auto merge of #15037 : zzmp/rust/doc/hotkeys, r=alexcrichton

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.