Add words about wildcard certificates to the cert CN setting

[alexeyzimarev]

https://developers.eventstore.com/server/v21.10/security/configuration.html#certificate-common-name

We need to explain that when using certificated signed by a trusted CA, the setting must be changed. Specifically mention wildcard certificates.

[mat-mcloughlin]

It would be good if we had a sample to go with this.