doc: make the description of how tcp_ecn works more explicit and clear

Make the description of how tcp_ecn works a bit more explicit and clear.
Signed-off-by: Rick Jones <rick.jones2@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -199,15 +199,16 @@ tcp_early_retrans - INTEGER
 	Default: 2
 
 tcp_ecn - INTEGER
-	Enable Explicit Congestion Notification (ECN) in TCP. ECN is only
-	used when both ends of the TCP flow support it. It is useful to
-	avoid losses due to congestion (when the bottleneck router supports
-	ECN).
+	Control use of Explicit Congestion Notification (ECN) by TCP.
+	ECN is used only when both ends of the TCP connection indicate
+	support for it.  This feature is useful in avoiding losses due
+	to congestion by allowing supporting routers to signal
+	congestion before having to drop packets.
 	Possible values are:
-		0 disable ECN
-		1 ECN enabled
-		2 Only server-side ECN enabled. If the other end does
-		  not support ECN, behavior is like with ECN disabled.
+		0 Disable ECN.  Neither initiate nor accept ECN.
+		1 Always request ECN on outgoing connection attempts.
+		2 Enable ECN when requested by incomming connections
+		  but do not request ECN on outgoing connections.
 	Default: 2
 
 tcp_fack - BOOLEAN