Figure out what OAPI and Kubernetes CEL stuff should be covered in XRD docs

### What's Missing?
https://kubernetes.io/blog/2022/09/29/enforce-immutability-using-cel/#generated-schema

We can make XRD fields immutable using the CEL stuff for CRDs.

Example in an XRD

```yaml
versions:
- name: v1alpha1
served: true
referenceable: true
schema:
openAPIV3Schema:
type: object
properties:
spec:
type: object
properties:
someData:
type: object
properties:
myField:
items:
type: string
type: array
x-kubernetes-validations:
- message: Value is immutable
rule: self == oldSelf
```

Trying to update the field will produce an error.