(doc) Change search placeholder text.

Update placeholder text to make keyboard shortcuts more apparent.
(doc) Use correct, normalized hotkeys in search.

`event.which` is a cross-browser normalized from jQuery, so this changes all instances of `e.keyCode` in `main.js` to `e.which`.

In addition, it fixes the code for `?`. Previously `188`, it was changed to `191`. `188` is the `,` or `<`.
(doc) Update shortcut menu with working shortcuts (tabs).
(doc) Changed height of search help.