Documentation: Clarify connection tracking tutorial

Documentation: Clarify connection tracking tutorial

The current documentation states that "all packets entering OVS for
the first time are "untracked"".  However there is a minor exception
to this in the case where a packet (re)enters the same datapath and
the namespace has not changed.  In that case there is no need to
scrub the packet and in this case the connection may already be
in the "tracked" state.

Reported-by: Quan Tian <qtian@vmware.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>