[TCP] FRTO: Update sysctl documentation

Since the SACK enhanced FRTO was added, the code has been
under test numerous times so remove "experimental" claim
from the documentation. Also be a bit more verbose about
the usage.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -180,13 +180,20 @@ tcp_fin_timeout - INTEGER
 	to live longer.	Cf. tcp_max_orphans.
 
 tcp_frto - INTEGER
-	Enables F-RTO, an enhanced recovery algorithm for TCP retransmission
+	Enables Forward RTO-Recovery (F-RTO) defined in RFC4138.
+	F-RTO is an enhanced recovery algorithm for TCP retransmission
 	timeouts.  It is particularly beneficial in wireless environments
 	where packet loss is typically due to random radio interference
-	rather than intermediate router congestion. If set to 1, basic
-	version is enabled. 2 enables SACK enhanced F-RTO, which is
-	EXPERIMENTAL. The basic version can be used also when SACK is
-	enabled for a flow through tcp_sack sysctl.
+	rather than intermediate router congestion.  FRTO is sender-side
+	only modification.  Therefore it does not require any support from
+	the peer, but in a typical case, however, where wireless link is
+	the local access link and most of the data flows downlink, the
+	faraway servers should have FRTO enabled to take advantage of it.
+	If set to 1, basic version is enabled.  2 enables SACK enhanced
+	F-RTO if flow uses SACK.  The basic version can be used also when
+	SACK is in use though scenario(s) with it exists where FRTO
+	interacts badly with the packet counting of the SACK enabled TCP
+	flow.
 
 tcp_frto_response - INTEGER
 	When F-RTO has detected that a TCP retransmission timeout was