This brings total updater coverage, as reported by 'guix refresh
--list-updaters', from 78% to 88.3%.  Among many other things, it covers
freedesktop.org packages.

* guix/gnu-maintenance.scm (html-updatable-package?)
(latest-html-updatable-release): New procedures.
(%generic-html-updater): New variable.
* doc/guix.texi (Invoking guix refresh): Document it.