vswitch: Clarify what "ingress" means in policing

vswitch: Clarify what "ingress" means in policing

The man page for ovs-vswitchd.conf explains how ingress policing works.
However, what "ingress" means is a bit confusing depending on the
perspective.  For vSwitch, it's from the switch's perspective.  This
means on a PIF, it's the rate traffic comes into the box.  On a VIF,
it's the rate traffic can be *transmitted* from a VM.  This commit
clarifies the man page a bit.

Thanks to Johan for pointing out the problem.