Automatic merge from submit-queue
api: types: document NodeSpec ID fields
Document MachineID as the preferred field from the discussion here:
https://groups.google.com/d/msg/kubernetes-sig-node/_3mXXB1ohZs/EuYtCzHvBgAJ
I couldn't get the generated things to update. I ran:
```
hack/update-generated-runtime.sh
hack/update-codegen.sh
hack/update-generated-swagger-docs.sh
hack/update-generated-protobuf.sh
hack/update-codecgen.sh
```
What did I miss?!
cc @dchen1107 @thockin