Document availability of xcursor-themes in the package description

and the x-cursor-theme alternative.

Don't set a package dependency, since you don't necessarily want a
cursor-theme package installed (core theme works automatically,
doesn't need the xcursor-themes package).  There is no standardised
virtual package x-cursor-theme to set a Suggests: x-cursor-theme (and
not enough need to create one and update all theme packages).

Closes: #860503.