Add godoc for targetRef CEL validation in BackendTLSPolicy

Signed-off-by: Norwin Schnyder <[email protected]>

apis/v1alpha3/backendtlspolicy_types.go

@@ -65,6 +65,15 @@ type BackendTLSPolicySpec struct {
 	// by default, but this default may change in the future to provide
 	// a more granular application of the policy.
 	//
+	// TargetRefs must be _distinct_. This means either that:
+	//
+	// * They select different objects. If this is the case, then targetRef
+	//   entries are distinct. In terms of fields, this means that the
+	//   multi-part key defined by `group`, `kind`, and `name` must
+	//   be unique across all targetRef entries in the BackendTLSPolicy.
+	// * They do not select different objects, each TargetRef that selects
+	//   the same object must set the sectionName to a different value.
+	//
 	// Support: Extended for Kubernetes Service
 	//
 	// Support: Implementation-specific for any other resource