xsk: Documentation for XDP_SHARED_UMEM between queues and netdevs

Add documentation for the XDP_SHARED_UMEM feature when a UMEM is
shared between different queues and/or netdevs.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/1598603189-32145-16-git-send-email-magnus.karlsson@intel.com