doc: announce ethdev API changes for callbacks
doc: announce ethdev API changes for callbacks

Ethdev APIs to add callback return the callback object as "void *",
update return type to actual object type
"struct rte_eth_rxtx_callback *"

Signed-off-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>
Acked-by: Konstantin Ananyev <cbd5212b59ab42a210d72992d23a3aa17cfd3eaf@intel.com>
Acked-by: Jerin Jacob <5bca7d2a432bc0f5422b70b2701a4e4fa32edd00@caviumnetworks.com>
Acked-by: Hemant Agrawal <90d96ef1ecd8ed49b97c7897be950e80824a9c5c@nxp.com>

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