Document that an AD realm must not be shared between multiple k8s clusters

[nightkr]

Followup to #402.

• Add note to docs about each K8s cluster requiring its own AD realm.
• Detect conflicts during AD provisioning, and link to the docs above.

[lfrancke]

None of the two checkboxes are ticked. Are they done?

[nightkr]

Yes, #408 covers both of them.