Update description of net.sctp.sctp_rmem and net.sctp.sctp_wmem tunables

sctp does not use second and third ("default" and "max") values
of sctp_rmem tunable. The format is the same as tcp_rmem
but the meaning is different so make the documentation explicit to
avoid confusion.

sctp_wmem is not used at all.

Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Max Matveev <makc@redhat.com>
Reviewed-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>