[docs] Document containers.*.envFrom

[joaomlneto]

The documentation currently only shows how to load single keys from Secrets and Configmaps using env.valueFrom.secretKeyRef and env.valueFrom.configMapKeyRef.

However, one can use envFrom.configMapRef and envFrom.secretRef to load all variables from a ConfigMap/Secret resp.:

yaml
      containers:
      - image: ${IMAGE}
        envFrom:
        - configMapRef:
            name: my-config-map