Extend webhook marker documentation

[anyasabo]

Extends the marker documentation for webhook generation for name and path.

The OpenAPI specs for webhooks require the name is a domain name.

The path is handled specially by controller-runtime, so this adds documentation on how controller-runtime handles it here:
https://github.com/kubernetes-sigs/controller-runtime/blob/master/pkg/builder/webhook.go#L158

Otherwise the webhook configuration generated by controller-tools will not match what actually runs with controller-runtime

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[anyasabo]

/assign @droot

[mengqiy]

Should be a domain with at least three segments separated by dots

Probably just say:
Name should be fully qualified, e.g. imagepolicy.kubebuilder.io.

[anyasabo]

An FQDN can also simply be kubebuilder.io, so this is more specific about the requirements I think. This is simply using the wording direct from the webhook validation. This is the error you receive if you try to apply a webhook with a name with < 3 labels:

The ValidatingWebhookConfiguration "validating-webhook-configuration" is invalid: webhooks[0].name: Invalid value: "funfunfun": should be a domain with at least three segments separated by dots

[mengqiy]

/lgtm
/approve

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