doc: announce dpaa-specific API parameter change

'port_id' storage size should be 'uint16_t', the API
'rte_pmd_dpaa_set_tx_loopback()' has it as 'uint8_t' but fixing it is an
ABI breakage, that is why planning the fix in v20.11 release where ABI
breakage is allowed.

Signed-off-by: Sachin Saxena <sachin.saxena@nxp.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Acked-by: Zhiyong Yang <zhiyong.yang@intel.com>