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.
