Documentation for this feature was missing from the patchset.
Copied a lot from the netdev 2.1 paper, addressing some small
interface changes since then.

Changes
  v1 -> v2
    - change email discussion URL format
    - clarify that u32 counter is per-syscall, unsigned and
      wraps after UINT_MAX calls
    - describe errno on send failure specific to MSG_ZEROCOPY
    - a few very minor rewordings
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>