net: correct docs of KeepAlive field in Dialer type
KeepAlive field used to report the wording "keep-alive period"
which may be misleading. This field does not represent the whole
TCP keepalive time, that is the inactivity period upon which one
endpoint starts probing the other end. But it acctually specifies
the keepalive interval, that is the time between two keepalive
probes.