Indicate label validity for each CoJP message

[Mališa Vučinić]

Ben Kaduk wrote (https://mailarchive.ietf.org/arch/msg/6tisch/uvht3lyqg63xz7QzneaAIEkuZNk):
Section 11.1

It feels a little unusual to have a consolidate registry for CoJP
parameters that are used as map labels across different messages, without
some indication of which map labels are valid in which messages.

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[Mališa Vučinić]

Fix Issue #63: Indicate label validity for different objects.

→ <<cset fc0971bea9f4>>