mbuf: fix Tx checksum offload API doc

mbuf: fix Tx checksum offload API doc

When introducing rte_eth_tx_prepare(), the constraints on checksum
pre-filling for Tx offloads were relaxed because implemented in
the PMDs with rte_net_intel_cksum_flags_prepare() helper.
As a consequence, these old requirements are removed for:
	- PKT_TX_OUTER_IP_CKSUM
	- PKT_TX_IP_CKSUM
	- PKT_TX_[L4]_CKSUM
	- PKT_TX_TCP_SEG

Not sure SCTP offload is properly implemented though.

A reference to rte_eth_tx_prepare() is added in rte_eth_tx_burst() doc.

Fixes: 609dd68ef14f ("mbuf: enhance the API documentation of offload flags")
Fixes: 4fb7e803eb1a ("ethdev: add Tx preparation")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Olivier Matz <olivier.matz@6wind.com>