qapi/net: Wean SLIRP off QAPI type String to improve documentation

String's doc comment is useless.  Replace its use in NetdevUserOptions
by identical types with hopefully useful documentation.

While there, add a non-doc comment explaining why the port forwarding
interface is problematic.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20260506105421.2461117-4-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
[Commit message typo fixed]
2 files changed