Documentation: fix sctp_wmem in ip-sysctl.rst

Since commit 1033990a ("sctp: implement memory accounting on tx path"),
SCTP has supported memory accounting on tx path where 'sctp_wmem' is used
by sk_wmem_schedule(). So we should fix the description for this option in
ip-sysctl.rst accordingly.

v1->v2:
  - Improve the description as Marcelo suggested.

Fixes: 1033990a ("sctp: implement memory accounting on tx path")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.rst

@@ -2866,7 +2866,14 @@ sctp_rmem - vector of 3 INTEGERs: min, default, max
 	Default: 4K
 
 sctp_wmem  - vector of 3 INTEGERs: min, default, max
-	Currently this tunable has no effect.
+	Only the first value ("min") is used, "default" and "max" are
+	ignored.
+
+	min: Minimum size of send buffer that can be used by SCTP sockets.
+	It is guaranteed to each SCTP socket (but not association) even
+	under moderate memory pressure.
+
+	Default: 4K
 
 addr_scope_policy - INTEGER
 	Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00