Document DomainClaims and administrator management of DomainMapping

[evankanderson]

What is changing? (Please include as many details as possible.)

Administrators can control which namespaces may use a custom domain for DomainMappings by setting autocreateClusterDomainClaims to false in the config-network ConfigMap, and then manually creating ClusterDomainClaim objects to assign domain names to specific namespaces.

This isn't documented anywhere, and I only picked it up by accident in a @markusthoemmes / @julz chat. 😁

How will this impact our users?

Administrators will have the ability to control which namespaces may use custom domains, and which custom domains may be used on the cluster at all. (This should go in "Administrator > Serving" guide somewhere reasonable.)

In what release will this take happen (to the best of your knowledge)?

v0.20 ?

Context

Link to associated PRs or issues from other repos here.

1. Add Namespace field to ClusterDomainClaim networking#326

Additional info

[julz]

I have it on my list to do this

/assign