TCP: update ip_local_port_range documentation

The explanation of ip_local_port_range in
Documentation/networking/ip-sysctl.txt contains several factual
errors:

- The default value of ip_local_port_range does not depend on the
  amount of memory available in the system.
- tcp_tw_recycle is not enabled by default.
- 1024-4999 is not the default value.
- Etc.

Clean up the mess.

Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>