Document the email policy in the reference

Author: MTRNord

docs/reference/configuration.md

@@ -357,6 +357,13 @@ policy:
       allow_missing_client_uri: false
       # don't require clients to provide a contacts field. default: false
       allow_missing_contacts: false
+      # restrict emails on registration to a specific domain
+      # This array is evaluated as a glob
+      allowed_domains:
+        - *.example.com
+      # ban specific domains from registration
+      banned_domains:
+        - *.banned.example.com
 ```
 
 ## `rate_limiting`