tcp: document tcp_max_ssthresh (Limited Slow-Start)

Base on Ilpo's patch about documenting tcp_max_ssthresh.
(see http://marc.info/?l=linux-netdev&m=117950581307310&w=2

)

According to errata of RFC3742, fix the number of segments increased
during RTT time.

Just to state the occasion to use this parameter, But
about how to set parameter value, maybe some others can do it.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>