docs: networking: convert proc_net_tcp.txt to ReST

- add SPDX header;
- add a document title;
- mark code blocks and literals as such;
- mark tables as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -94,6 +94,7 @@ Contents:
    pktgen
    plip
    ppp_generic
+   proc_net_tcp
 
 .. only::  subproject and html
 

Documentation/networking/proc_net_tcp.rst

@@ -1,3 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============================================
+The proc/net/tcp and proc/net/tcp6 variables
+============================================
+
 This document describes the interfaces /proc/net/tcp and /proc/net/tcp6.
 Note that these interfaces are deprecated in favor of tcp_diag.
 
@@ -7,7 +13,7 @@ and tcp6_seq_show() 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):
+up into 3 parts because of the length of the line)::
 
    46: 010310AC:9C4C 030310AC:1770 01
    |      |      |      |      |   |--> connection state
@@ -40,9 +46,12 @@ up into 3 parts because of the length of the line):
     |---------------------------------------------> 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
+ ==  ================================================================