It's common for search boxes like ours to automatically navigate to
the first search result when the `enter` / `select` key is pressed, as
that can allow for rapid navigation. E.g., the MDN documentation does
this.
Let's similarly navigate to the first result when, in the search box,
the user presses the `enter` key and there is a first result to which
to navigate.
If, when searching, one pressed the down arrow key when there were no
results, this caused an uncaught exception and defocused the search
box.
Let's prevent this and keep the search box focused when pressing down
in this state by checking first whether there is a result for us to
focus instead.
We allow for using `s` to open the search box, but it's more common to
use `/` (forward slash) for this. E.g., MDN's documentation uses `/`
for search. Rustdoc and GitHub accept either.
Let's allow either key to be used, and let's switch to "advertising"
`/` rather than `s` in the hover text for the search button.
In making that switch, let's also simplify that hover text a bit.
Previously it had said "Search. (Shortkey: s)". This was the only top
button on which we had included a period in the hover text. Let's
remove that, and let's remove the "shortkey" bit of jargon. It's
enough to just put `/` in a parenthetical, i.e. "Search (`/`)".
People will gleam from that what we mean.
We've also updated the guide accordingly.
