Bug 18811: [QA Follow-up] Resolved POD warnings

Bug 18811: [QA Follow-up] Resolved POD warnings

Trivial changes to silence qa tools on POD warnings for Subfields and Tags.

Note: Since Subfield and Tag only contain one sub which is considered as
private by Pod::Coverage, these modules are listed as unrated (no public
symbols defined) and trigger a FAIL on pod coverage in qa tools. This fail
can be ignored.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>