[TCP]: Add /proc/net/tcp{,6} layout documentation.

Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/proc_net_tcp.txt

+This document describes the interfaces /proc/net/tcp and /proc/net/tcp6.
+
+These /proc interfaces provide information about currently active TCP 
+connections, and are implemented by tcp_get_info() in net/ipv4/tcp_ipv4.c and
+tcp6_get_info() in net/ipv6/tcp_ipv6.c, respectively.
+
+It will first list all listening TCP sockets, and next list all established
+TCP connections. A typical entry of /proc/net/tcp would look like this (split 
+up into 3 parts because of the length of the line):
+
+   46: 010310AC:9C4C 030310AC:1770 01 
+   |      |      |      |      |   |--> connection state
+   |      |      |      |      |------> remote TCP port number
+   |      |      |      |-------------> remote IPv4 address
+   |      |      |--------------------> local TCP port number
+   |      |---------------------------> local IPv4 address
+   |----------------------------------> number of entry
+
+   00000150:00000000 01:00000019 00000000  
+      |        |     |     |       |--> number of unrecovered RTO timeouts
+      |        |     |     |----------> number of jiffies until timer expires
+      |        |     |----------------> timer_active (see below)
+      |        |----------------------> receive-queue
+      |-------------------------------> transmit-queue
+
+   1000        0 54165785 4 cd1e6040 25 4 27 3 -1
+    |          |    |     |    |     |  | |  | |--> slow start size threshold, 
+    |          |    |     |    |     |  | |  |      or -1 if the treshold
+    |          |    |     |    |     |  | |  |      is >= 0xFFFF
+    |          |    |     |    |     |  | |  |----> sending congestion window
+    |          |    |     |    |     |  | |-------> (ack.quick<<1)|ack.pingpong
+    |          |    |     |    |     |  |---------> Predicted tick of soft clock
+    |          |    |     |    |     |              (delayed ACK control data)
+    |          |    |     |    |     |------------> retransmit timeout
+    |          |    |     |    |------------------> location of socket in memory
+    |          |    |     |-----------------------> socket reference count
+    |          |    |-----------------------------> inode
+    |          |----------------------------------> unanswered 0-window probes
+    |---------------------------------------------> uid
+
+timer_active:
+  0  no timer is pending
+  1  retransmit-timer is pending
+  2  another timer (e.g. delayed ack or keepalive) is pending
+  3  this is a socket in TIME_WAIT state. Not all fields will contain 
+     data (or even exist)
+  4  zero window probe timer is pending