doc: announce adoption of C11 atomic operations semantics

doc: announce adoption of C11 atomic operations semantics

As agreed in the DPDK tech board [1], after 20.05 release, patches must
use C11 atomic operations semantics with the help of wrappers.

[1] http://mails.dpdk.org/archives/dev/2020-April/165143.html

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: David Christensen <drc@linux.vnet.ibm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>