mbuf: update Tx VLAN and QinQ flags documentation

Currently PKT_TX_VLAN and PKT_TX_QINQ mbuf flags are documented as
they are to say packet contains VLAN or QINQ information.

Updating the definition as they are requests from application to
driver to insert VLAN or double VLAN tags into packet.

Fixes: dc6c911c9993 ("mbuf: use reserved space for double vlan")
Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>