Data draft: Consider additional verbiage about node-id uniqueness

[brockners]

https://mailarchive.ietf.org/arch/msg/ippm/v4a75IXTi8GKfDl_Ddm_5WN6gxs

"While, for example, the
IOAM node identifier (Node-ID) does not have to be unique in a
deployment , the combination of Node-ID and Namespace-ID will
always be unique.
"
Why assume that the node id will not be unique? It is only to increase complexity.The uniqueness should be guaranteed which is common in other protocols design such as LSR ID in MPLS signalling, Router ID in IGP, etc.

[brockners]

Addressed in #119