Generate docstrings for CRD Spec and Status fields

Crossplane's CRDs [use the `private/model/api.Shape.Documentation()` method](https://github.com/aws-controllers-k8s/code-generator/blob/38c3b3ffb70cc4d86ae7825a22fc30fa0f16f034/templates/crossplane/apis/crd.go.tpl#L25-L27) to generate a docstring for each field in the Spec and Status structs. ACK's CRDs should have the same.