doc: announce renaming of mbuf offload flags

doc: announce renaming of mbuf offload flags

The mbuf offload flags do not match the DPDK namespace (they are
not prefixed by RTE_). Announce their rename in 21.11, and the
removal of the old names in 22.11.

A draft coccinelle script is provided to anticipate what the
renaming will be.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>