update note on commonName field

See cert-manager#1474

Signed-off-by: Ashley Davis <[email protected]>

content/docs/usage/certificate.md

@@ -83,8 +83,9 @@ spec:
   subject:
     organizations:
       - jetstack
-  # The use of the common name field has been deprecated since 2000 and is
-  # discouraged from being used.
+  # Avoid using commonName for end-entity certificates for DNS names. Unless you have a specific
+  # need for it in your environment, use dnsNames exclusively to avoid the issues with commonName.
+  # Most commonly, commonName is used to give human-readable names to CA certificates.
   commonName: example.com
 
   # The literalSubject field is exclusive with subject and commonName. It allows