doc: add external pinned memory mbuf to release notes
doc: add external pinned memory mbuf to release notes

Update release notes features for the new mbuf pool
with external pinned buffers.

Signed-off-by: Viacheslav Ovsiienko <5b8db9bb93a9a9aefc8828f79604d1fe963a53dd@mellanox.com>

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