macsec: add user manual description for extended packet number feature

macsec: add user manual description for extended packet number feature

Update the user manual describing how to use extended packet number (XPN)
feature for macsec. As part of configuring XPN, providing ssci and salt is
required hence update user manual on  how to provide the above as part of
the ip macsec command.

Signed-off-by: Emeel Hakim <ehakim@nvidia.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David Ahern <dsahern@kernel.org>