Explain why device plugin resources are different from cpu/memory

requestResource function makes several assumptions about values that
limits and requests sections of pod spec can have for device plugin
managed resources. It's not immediately clear why we may not want to
handle these resources in exactly the same way as other resources
(memory, cpu) in regards to QoS.

This patch explains the reasoning behind the difference, with links to
relevant design documents, in hope future reader will be less puzzled.

pkg/virt-controller/services/template.go

@@ -167,6 +167,30 @@ func CPUModelLabelFromCPUModel(vmi *v1.VirtualMachineInstance) (label string, er
 	return
 }
 
+// Request a resource by name. This function bumps the number of resources,
+// both its limits and requests attributes.
+//
+// If we were operating with a regular resource (CPU, memory, network
+// bandwidth), we would need to take care of QoS. For example,
+// https://kubernetes.io/docs/tasks/configure-pod-container/quality-service-pod/#create-a-pod-that-gets-assigned-a-qos-class-of-guaranteed
+// explains that when Limits are set but Requests are not then scheduler
+// assumes that Requests are the same as Limits for a particular resource.
+//
+// But this function is not called for this standard resources but for
+// resources managed by device plugins. The device plugin design document says
+// the following on the matter:
+// https://github.com/kubernetes/community/blob/master/contributors/design-proposals/resource-management/device-plugin.md#end-user-story
+//
+// ```
+// Devices can be selected using the same process as for OIRs in the pod spec.
+// Devices have no impact on QOS. However, for the alpha, we expect the request
+// to have limits == requests.
+// ```
+//
+// Which suggests that, for resources managed by device plugins, 1) limits
+// should be equal to requests; and 2) QoS rules do not apply.
+//
+// Hence we don't copy Limits value to Requests if the latter is missing.
 func requestResource(resources *k8sv1.ResourceRequirements, resourceName string) {
 	name := k8sv1.ResourceName(resourceName)