Better docs for `UniqueItems`

[guettli]

Afaik the kubebuilder docs are autogenerated from this repo.

I would like to improve the docs about UniqueItems

https://book.kubebuilder.io/reference/markers/crd-validation.html#crd-validation

I get:

Forbidden: uniqueItems cannot be set to true since the runtime complexity becomes quadratic

---

I would like to add the answer to this question to the docs:

What alternatives does a CRD author have (except writing a web hook)?

Example CRD:

type FooSpec struct {
    // +kubebuilder:validation:UniqueItems=true
    MyItems []string `json:"myItems"`