A11Y improvement: Empty result text is not read by screenreaders

In this patch i've added a unique ID to the empty-result div.
That ID is referenced by the new "aria-describedby" on the search input field.

I've also updated the description of the field in the admin form.
It used to be:
"Enter the HTML to show when the search input gets focus and there is no search term in the input. Useful for "quick links" for instance."

I've changed it to:
"When the search input gets focus and there is no search term, this text will be shown and is read by screen readers. Keep this short and do not use HTML to avoid accessibility failures."

This field has been 'abused' a bit to show some quicklinks, but those links were never accessible to screen-readers (screen reader users didn't even know that text existed) and those links are not reachable by keyboard users.

So it's better to update the description for it's modern use.
Showing a list of quicklinks (when you haven't entered a search term yet) that's accessible should be built completely different.

This patch shouldn't 'break' any existing sites using it for a list of links, it just exposes the problem they already had.

Patch applied and works like intended.