Update availability set documentation

docs/book/src/topics/failure-domains.md

@@ -188,7 +188,7 @@ When cluster api detects that the region has no failure domains, it creates avai
 The availability sets created are as follows:
 
 1. For control plane vms, an availability set will be created and suffixed with the string "control-plane".
-2. For Worker node vms, an availability set will be created for each machine deployment, and suffixed with the machine deployment name.
+2. For Worker node vms, an availability set will be created for each machine deployment/set, and suffixed with the machine deployment/set name. Important note: make sure that machine deployment `Spec.Template.Labels` includes `"cluster.x-k8s.io/deployment-name"` label, it will not have this label by default if machine deployment was created with a custom `Spec.Selector.MatchLabels` field. Machine set should `Spec.Template.Labels` include `"cluster.x-k8s.io/set-name"`.
 
 Consider the following cluster configuration: