Document canonical way to set priority for static pods

Per doc, static pods does not support priorityClassName field. However, when static pods are created with only priority values set to 2000001000 (that matches with system-node-critical) kubelet prints the following error:

forbidden: the integer value of priority (2000001000) must not be provided in pod spec; priority admission controller computed 0 from the given PriorityClass name" pod="kube-system/...