Add a description and default value. Switch the documentation to mention the
service-type and the configuration record, rather than the upower-service
procedure.

* gnu/services/desktop.scm (upower-service-type)[description, default-value]:
Define these fields.
(%desktop-services): Change (upower-service) to (service upower-service-type).
* doc/guix.texi (Desktop Services): Update the upower service documentation.