doc: announce common prefix for ethdev

doc: announce common prefix for ethdev

Announce adding 'RTE_ETH_' prefix to all public ethdev macros/enums on
v21.11.
Backward compatibility macros will be added on v21.11 and they will be
removed on v22.11.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.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: Raslan Darawsheh <rasland@nvidia.com>
Acked-by: Tyler Retzlaff <roretzla@linux.microsoft.com>