doc: update backport guidelines
doc: update backport guidelines

Updating the docs to elaborate on the stable release
characteristics and better document the current practice
about new features in stable releases.

Signed-off-by: Kevin Traynor <1f46c93815774a2e602547cf1e37c682d62b566d@redhat.com>
Acked-by: Luca Boccassi <020748dc039465edc3ee6f30c530b21164cdce27@debian.org>
Acked-by: Christian Ehrhardt <48da28e2c2cd455d60e19b14c46925e20a3c5d5b@canonical.com>

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