Add note that CommonName is not looked at when any SAN is set

Signed-off-by: Maartje Eyskens <maartje@eyskens.me>

pkg/apis/certmanager/v1alpha2/types_certificate.go

@@ -79,6 +79,7 @@ type CertificateSpec struct {
 	// CommonName is a common name to be used on the Certificate.
 	// The CommonName should have a length of 64 characters or fewer to avoid
 	// generating invalid CSRs.
+	// This value is ignored by TLS clients when any subject alt name is set.
 	// +optional
 	CommonName string `json:"commonName,omitempty"`