doc: clarify L3 Tx checksum prerequisite
doc: clarify L3 Tx checksum prerequisite

Based on PKT_TX_IP_CKSUM definition the user needs
to fill l2_len and l3_len mbuf fields before issuing
HW Tx checksum request.

Fixes: dad1ec72a377 ("doc: document NIC features")
Cc: stable@dpdk.org

Signed-off-by: Jerin Jacob <5bca7d2a432bc0f5422b70b2701a4e4fa32edd00@caviumnetworks.com>
Acked-by: Andrew Rybchenko <ac94ab2a8fe9a9a087f6e24dcbc16626b52c07e8@solarflare.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk