qat: improve documentation related to device provisioning with max-num-devices/allocation-policy parameters

We've added #898 and #884 but there are problems with the user experience when combining them. We need better documentation to explain the shortcomings. The biggest problem is:

If the initcontainer is used to provision the devices and `-max-num-devices` is set to a low value (e.g., < 16), all the VFs are from the same PF. This means `balanced` policy is useless.