services: Deprecate 'xfce-desktop-service'.

* gnu/services/desktop.scm (xfce-desktop-service-type)[default-value]
[description]: New fields.
(xfce-desktop-service): Deprecate.
* gnu/system/examples/desktop.tmpl: Use the (service …) form.
* gnu/installer/services.scm (%desktop-environments): Add TODO comment.
* doc/guix.texi (Desktop Services): Adjust accordingly, and fix spelling
of "Xfce" throughout.