systemd: Add Documentation= to location-update

systemd: Add Documentation= to location-update

Systemd units are expected to provide some documentation information
such as manpages, or direct links, which provide more details about
that unit. This commit simply links location-update.service to the
manual for location(8) followed by a fallback to the online manual.

See: https://manpages.debian.org/buster/systemd/systemd.unit.5.en.html#%5BUNIT%5D_SECTION_OPTIONS

Resolves: lintian: systemd-service-file-missing-documentation-key

Signed-off-by: Valters Jansons <valter.jansons@gmail.com>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>