[Feedback]: tidying up the kerberos installation page

### Page URL

https://documentation.ubuntu.com/server/how-to/kerberos/install-a-kerberos-server/

### Select an option

- [ ] I found what I was looking for
- [ ] I couldn't find what I was looking for
- [ ] I found the information, but it was incorrect
- [x] I found the information, but it was incomplete
- [ ] I found the information, but it was confusing
- [ ] I encountered a technical issue (e.g., broken link, image not loading)

### Issue Description

a) Tiny edit under "Configure the Kerberos server"
> If you need to adjust the KDC settings
Make that "If you ever need to adjust the KDC settings", otherwise it might imply that something needs to be changed now, at this stage, and that's not the case.

b) addprinc ubuntu output
The output is slightly different. Normally I wouldn't bother, but since it's dropping an all-capital WARNING, perhaps we could update it. New output:
```
Authenticating as principal root/admin@LXD with password.
kadmin.local: addprinc ubuntu
No policy specified for ubuntu@LXD; defaulting to no policy <------
Enter password for principal "ubuntu@LXD":
Re-enter password for principal "ubuntu@LXD":
Principal "ubuntu@LXD" created.
kadmin.local:
```
In the output above I used the LXD realm/domain, but the docs use EXAMPLE.COM, which we should keep.

c) The introduction at the top mentions a secondary kdc that the doc never installs. It should be removed, as there is another page that explains how to install the secondary.

d) The meta text should also be updated, because it also mentions a secondary:
` description: Install and configure an MIT Kerberos server on Ubuntu with primary and secondary KDC setup, including DNS and NTP prerequisites.`

Use the page URL to locate the Kerberos installation page source and inspect its introduction, “Configure the Kerberos server” section, addprinc output example, and metadata description. Update the requested wording and example, remove references to secondary KDC installation, and verify the documentation build or preview shows the revised page.

Written by the indexing model from the issue text.