doc: announce removal of mbuf legacy refcnt field

refcnt_atomic member in structures rte_mbuf and rte_mbuf_ext_shared_info
will be removed in 20.11 release.

Suggested-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Signed-off-by: Phil Yang <phil.yang@arm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: David Marchand <david.marchand@redhat.com>