services: elogind: Fix default behavior for lid close.

services: elogind: Fix default behavior for lid close.

Fixes <https://issues.guix.gnu.org/57052>, which was a behavior change
introduced inadvertently in 4c698cd51209a0102477478b026ea04bd7e45908.

* gnu/services/desktop.scm (<elogind-configuration>)
[handle-lid-switch-external-power]: Default to *unspecified*, which serializes
to nothing.  This matches upstream behavior, meaning that even when plugged to
a power cord, a laptop will suspend when the lid is closed.
* doc/guix.texi (Desktop Services): Update doc.

Reported-by: Cairn <cairn@pm.me>