xsk: document ability to redirect to any socket bound to the same umem

Document the ability to redirect to any socket bound to the same umem.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20240205123553.22180-3-magnus.karlsson@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>