[TCP]: Document several sysctls.

This adds documentation for tcp_moderate_rcvbuf, tcp_no_metrics_save,
tcp_base_mss, and tcp_mtu_probing.

Signed-off-by: John Heffner <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/ip-sysctl.txt

···
 	More congestion control algorithms may be available as modules,
 	but not loaded.
 
+tcp_base_mss - INTEGER
+	The initial value of search_low to be used by Packetization Layer
+	Path MTU Discovery (MTU probing).  If MTU probing is enabled,
+	this is the inital MSS used by the connection.
+
 tcp_congestion_control - STRING
 	Set the congestion control algorithm to be used for new
 	connections. The algorithm "reno" is always available, but
···
 
 	Defaults are calculated at boot time from amount of available
 	memory.
+
+tcp_moderate_rcvbuf - BOOLEAN
+	If set, TCP performs receive buffer autotuning, attempting to
+	automatically size the buffer (no greater than tcp_rmem[2]) to
+	match the size required by the path for full throughput.  Enabled by
+	default.
+
+tcp_mtu_probing - INTEGER
+	Controls TCP Packetization-Layer Path MTU Discovery.  Takes three
+	values:
+	  0 - Disabled
+	  1 - Disabled by default, enabled when an ICMP black hole detected
+	  2 - Always enabled, use initial MSS of tcp_base_mss.
+
+tcp_no_metrics_save - BOOLEAN
+	By default, TCP saves various connection metrics in the route cache
+	when the connection closes, so that connections established in the
+	near future can use these to set initial conditions.  Usually, this
+	increases overall performance, but may sometimes cause performance
+	degredation.  If set, TCP will not cache metrics on closing
+	connections.
 
 tcp_orphan_retries - INTEGER
 	How may times to retry before killing TCP connection, closed