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: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NMax Matveev <makc@redhat.com>
Reviewed-by: NShan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1455,10 +1455,17 @@ sctp_mem - vector of 3 INTEGERs: min, pressure, max
 	Default is calculated at boot time from amount of available memory.
 
 sctp_rmem - vector of 3 INTEGERs: min, default, max
-	See tcp_rmem for a description.
+	Only the first value ("min") is used, "default" and "max" are
+	ignored.
+
+	min: Minimal size of receive buffer used by SCTP socket.
+	It is guaranteed to each SCTP socket (but not association) even
+	under moderate memory pressure.
+
+	Default: 1 page
 
 sctp_wmem  - vector of 3 INTEGERs: min, default, max
-	See tcp_wmem for a description.
+	Currently this tunable has no effect.
 
 addr_scope_policy - INTEGER
 	Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00