ovs-ctl: Add comment to explain why we only save ofports for pre-1.10.

We've had a couple of questions about this lately, including one suggestion
that we should always save the OpenFlow port numbers.  This explains why
the behavior is as it is.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>