OCPBUGS-41971-ent18: Added note to the Ingress capability docs

Author: dfitzmau

modules/ingress-operator.adoc

@@ -20,9 +20,12 @@ ifdef::cluster-caps[= Ingress Capability]
 == Purpose
 
 ifdef::cluster-caps[]
+The Ingress Operator provides the features for the Ingress Capability. The Ingress Capability is enabled by default.
 
-The Ingress Operator provides the features for the `Ingress` capability.
-
+[IMPORTANT]
+====
+If you set the `baselineCapabilitySet` field to `None`, you must explicitly enable the Ingress Capability, because the installation of a cluster fails if the Ingress Capability is disabled.
+====
 endif::cluster-caps[]
 
 The Ingress Operator configures and manages the {product-title} router.