Text & section references to [I-D.ietf-anima-constrained-voucher] are not up to date anymore

[EskoDijk]

For example, it refers to [I-D.ietf-anima-constrained-voucher] for discovery details but this I-D doesn't contain the details for CoAP, nor GRASP details. Rather it points to constrained-join-proxy to provide the details.

Reason: it's the JP that has to do the discovery of the Registrar, so it was assumed details would be in the JP document.