add notes regarding kerberos authentication for services

Co-authored-by: Copilot <copilot@github.com>

docs/admin-guide/configuration/basic/kerberos.md

@@ -8,7 +8,8 @@ By default only the real hostname is known to the AD and the system keytab.
 If a host is known in DNS with additional hostnames, then Kerberos does not work if these are used.
 
 Below configuration will add the respective aliases to the AD and the system keytab:
-```
+
+```yaml
 kerberos::aliases:
   - 'login.psi.ch'
   - 'llc.psi.ch'
@@ -20,7 +21,11 @@ Note that the short name without the domain part is automatically added.
 
 Also additional Kerberos Services might be introduced with the Hiera key `kerberos::additional_services`:
 
-```
+```yaml
 kerberos::additional_services:
   - 'http'
 ```
+
+By doing so, the respective service principal will be added to the AD and the system keytab. The service keytab should then be extracted from the system keytab and used for the respective service, e.g. Apache HTTPD.
+
+The ownership and permissions of the extracted service keytab should be set appropriately to the user running the service to ensure security. Afterwards the service can be configured to use the extracted keytab for e.g. authentication.