Add explanation for "CN" (#16714)

* Add explanation for "CN" (#16456)

* Grammar nit

content/en/docs/concepts/cluster-administration/certificates.md

@@ -25,7 +25,8 @@ manually through `easyrsa`, `openssl` or `cfssl`.
         tar xzf easy-rsa.tar.gz
         cd easy-rsa-master/easyrsa3
         ./easyrsa init-pki
-1.  Generate a CA. (`--batch` set automatic mode. `--req-cn` default CN to use.)
+1.  Generate a new certificate authority (CA). `--batch` sets automatic mode;
+    `--req-cn` specifies the Common Name (CN) for the CA's new root certificate.
 
         ./easyrsa --batch "--req-cn=${MASTER_IP}@`date +%s`" build-ca nopass
 1.  Generate server certificate and key.

content/en/docs/concepts/services-networking/ingress.md

@@ -319,8 +319,8 @@ type: kubernetes.io/tls
 
 Referencing this secret in an Ingress tells the Ingress controller to
 secure the channel from the client to the load balancer using TLS. You need to make
-sure the TLS secret you created came from a certificate that contains a CN
-for `sslexample.foo.com`.
+sure the TLS secret you created came from a certificate that contains a Common
+Name (CN), also known as a Fully Qualified Domain Name (FQDN) for `sslexample.foo.com`.
 
 ```yaml
 apiVersion: networking.k8s.io/v1beta1