packet: doc: add documentation for VLAN TPID delivery

Introduce TP_STATUS_VLAN_TPID_VALID bit into the documentation.

Signed-off-by: Atzm Watanabe <atzm@stratosphere.co.jp>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>