Document topologySpreadConstraint duplicate value conflict handling

[intUnderflow]

Description

TopologySpreadConstraints cannot conflict on a given (topologyKey, whenUnsatisfiable) pair, but this is currently not documented, this adds a note describing this behaviour.

You can see the limitation yourself by trying to create this deployment:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      topologySpreadConstraints:
      - maxSkew: 1
        topologyKey: "kubernetes.io/hostname"
        whenUnsatisfiable: "DoNotSchedule"
        labelSelector:
          matchLabels:
            app: nginx
      - maxSkew: 3
        topologyKey: "kubernetes.io/hostname"
        whenUnsatisfiable: "DoNotSchedule"
        labelSelector:
          matchLabels:
            foo: bar
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80

The wording on this is a bit dense, so happy for suggestions on how to make it clearer.

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[intUnderflow]

/sig docs

[sftim]

/retitle Document topologySpreadConstraint duplicate value conflict handling

[sftim]

/priority backlog
/triage accepted