Add docs on Election ID to Multiple Ingress Controller guide (#8855)

docs/index.md

@@ -228,3 +228,13 @@ If you start Ingress-Nginx B with the command line argument `--watch-ingress-wit
   --set controller.ingressClassByName=true
   ```
 - If you need to install yet another instance, then repeat the procedure to create a new namespace, change the values such as names & namespaces (for example from "-2" to "-3"), or anything else that meets your needs.
+- If you need to install all instances in the same namespace, then you need to specify a different **election id**, like this:
+  ```
+  helm install ingress-nginx-2 ingress-nginx/ingress-nginx  \
+  --namespace kube-system \
+  --set controller.electionID=nginx-two-leader \
+  --set controller.ingressClassResource.name=nginx-two \
+  --set controller.ingressClassResource.controllerValue="example.com/ingress-nginx-2" \
+  --set controller.ingressClassResource.enabled=true \
+  --set controller.ingressClassByName=true
+  ```

docs/user-guide/multiple-ingress.md

@@ -8,7 +8,7 @@ To fix this problem, use [IngressClasses](https://kubernetes.io/docs/concepts/se
 
 If all ingress controllers respect IngressClasses (e.g. multiple instances of ingress-nginx v1.0), you can deploy two Ingress controllers by granting them control over two different IngressClasses, then selecting one of the two IngressClasses with `ingressClassName`.
 
-First, ensure the `--controller-class=` and `--ingress-class` are set to something different on each ingress controller:
+First, ensure the `--controller-class=` and `--ingress-class` are set to something different on each ingress controller, If your additional ingress controller is to be installed in a namespace, where there is/are one/more-than-one ingress-nginx-controller(s) already installed, then you need to specify a different unique `--election-id` for the new instance of the controller.
 
 ```yaml
 # ingress-nginx Deployment/Statefulset
@@ -19,6 +19,7 @@ spec:
          - name: ingress-nginx-internal-controller
            args:
              - /nginx-ingress-controller
+             - '--election-id=ingress-controller-leader'
              - '--controller-class=k8s.io/internal-ingress-nginx'
              - '--ingress-class=k8s.io/internal-nginx'
             ...
@@ -53,6 +54,7 @@ or if installing with Helm:
 
 ```yaml
 controller:
+  electionID: ingress-controller-leader
   ingressClassResource:
     name: internal-nginx  # default: nginx
     enabled: true