doc: update API/ABI policy
doc: update API/ABI policy

Following agreement at the DPDK Technical Board meeting of 2017-10-13 [1],
update the documentation with the ABI/API policy changes.

[1] http://dpdk.org/ml/archives/dev/2017-October/079961.html

Signed-off-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>
Acked-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>
Acked-by: Yuanhan Liu <5219796f50b7b6a5f4ac3e8f0a0f9501ee64a87c@fridaylinux.org>
Acked-by: Kevin Traynor <1f46c93815774a2e602547cf1e37c682d62b566d@redhat.com>
Acked-by: Thomas Monjalon <5f50a84c1fa3bcff146405017f36aec1a10a9e38@monjalon.net>

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