doc: Tighten-up and clarify description of tcp_fin_timeout

The description for tcp_fin_timeout should be tigher and more clear.

In addition to being tighter, we should make the spelling of the
state name consistent with what utilities report, remove the now
dated reference to 2.2 and put the default in the consistent place.

Signed-off-by: Rick Jones <rick.jones2@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>