Add some docs

Author: MarcelMue

docs/book/src/developer/architecture/controllers/support-multiple-instances.md

@@ -24,6 +24,7 @@ in case the above limitations/extra complexity are acceptable for them.
 In order to make it possible for users to deploy multiple instances of the same provider:
 
 - Providers MUST support the `--namespace` flag in their controllers.
+- Providers MUST support the `--watch-filter` flag in their controllers.
 
 ⚠️ Users selecting this deployment model, please be aware:
 

docs/book/src/developer/providers/v1alpha3-to-v1alpha4.md

@@ -54,3 +54,28 @@ see [Multi-tenancy](../architecture/controllers/multi-tenancy.md) and [Support m
 more details.
 
 Specific changes related to this topic will be detailed in this document.
+
+## Support the cluster.x-k8s.io/watch-filter label and watch-filter flag.
+
+- A new label `cluster.x-k8s.io/watch-filter` provides the ability to filter the controllers to only reconcile objects with a specific label.
+- A new flag `watch-filter` enables users to specify the label value for the `cluster.x-k8s.io/watch-filter` label on controller boot.
+- The flag which enables users to set the flag value can be structured like this:
+  ```go
+  	fs.StringVar(&watchFilterValue, "watch-filter", "", fmt.Sprintf("Label value that the controller watches to reconcile cluster-api objects. Label key is always %s. If unspecified, the controller watches for all cluster-api objects.", clusterv1.WatchLabel))
+  ```
+- The `ResourceNotPausedAndHasFilterLabel` predicate is a useful helper to check for the pause annotation and the filter label easily:
+  ```go
+  c, err := ctrl.NewControllerManagedBy(mgr).
+		For(&clusterv1.MachineSet{}).
+		Owns(&clusterv1.Machine{}).
+		Watches(
+			&source.Kind{Type: &clusterv1.Machine{}},
+			handler.EnqueueRequestsFromMapFunc(r.MachineToMachineSets),
+		).
+		WithOptions(options).
+		WithEventFilter(predicates.ResourceNotPausedAndHasFilterLabel(ctrl.LoggerFrom(ctx), r.WatchFilterValue)).
+		Build(r)
+	if err != nil {
+		return errors.Wrap(err, "failed setting up with a controller manager")
+	}
+  ```