Provide better documentation on watching namespaces

[tomkerkhove]

Signed-off-by: Tom Kerkhove kerkhove.tom@gmail.com

Improvement for kedacore/keda#1377

Without:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: keda-operator
  namespace: platform-gateway-g3-dev
  labels:
    app: keda-operator
    name: keda-operator
    <redacted>
spec:
  replicas: 1
  selector:
    matchLabels:
      app: keda-operator
  template:
    metadata:
      labels:
        app: keda-operator
    spec:
      serviceAccountName: keda-operator
      securityContext:
        {}
      containers:
        - name: keda-operator
          securityContext:
            {}
          image: "docker.io/kedacore/keda:2.0.0"
          <redacted>
          env:
            - name: WATCH_NAMESPACE
              value: ""
            - name: POD_NAME
              valueFrom:
                fieldRef:
                  fieldPath: metadata.name
            - name: OPERATOR_NAME
              value: keda-operator
          volumeMounts:
          resources:
            {}
      volumes:
---
# Source: keda/templates/22-metrics-deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: keda-operator-metrics-apiserver
  namespace: platform-gateway-g3-dev
  labels:    
    <redacted>
spec:
  replicas: 1
  selector:
    matchLabels:
      app: keda-operator-metrics-apiserver
  template:
    metadata:
      labels:
        app: keda-operator-metrics-apiserver
      annotations:
    spec:
      serviceAccountName: keda-operator
      securityContext:
        {}
      containers:
        - name: keda-operator-metrics-apiserver
          securityContext:
            {}
          image: "docker.io/kedacore/keda-metrics-apiserver:2.0.0"
          <redacted>
          env:
            - name: WATCH_NAMESPACE
              value: ""

With:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: keda-operator
  namespace: platform-gateway-g3-dev
  labels:
    app: keda-operator
    name: keda-operator
    <redacted>
spec:
  replicas: 1
  selector:
    matchLabels:
      app: keda-operator
  template:
    metadata:
      labels:
        app: keda-operator
    spec:
      serviceAccountName: keda-operator
      securityContext:
        {}
      containers:
        - name: keda-operator
          securityContext:
            {}
          image: "docker.io/kedacore/keda:2.0.0"
          <redacted>
          env:
            - name: WATCH_NAMESPACE
              value: "foo"
            - name: POD_NAME
              valueFrom:
                fieldRef:
                  fieldPath: metadata.name
            - name: OPERATOR_NAME
              value: keda-operator
          volumeMounts:
          resources:
            {}
      volumes:
---
# Source: keda/templates/22-metrics-deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: keda-operator-metrics-apiserver
  namespace: platform-gateway-g3-dev
  labels:    
    <redacted>
spec:
  replicas: 1
  selector:
    matchLabels:
      app: keda-operator-metrics-apiserver
  template:
    metadata:
      labels:
        app: keda-operator-metrics-apiserver
      annotations:
    spec:
      serviceAccountName: keda-operator
      securityContext:
        {}
      containers:
        - name: keda-operator-metrics-apiserver
          securityContext:
            {}
          image: "docker.io/kedacore/keda-metrics-apiserver:2.0.0"
          <redacted>
          env:
            - name: WATCH_NAMESPACE
              value: "foo"