Review and Document vocabulary id determination

[rob-metalinkage]

links require the vocabulary identifier to be specified - this appears to be the last path entry in the concept URI?
This seems to be fragile in the face of duplicate path endings, even if URI are unique?

[nicholascar]

Actually it's a convenience thing when you see the vocab_id variable in the path of the object endpoint or in /vocabs/{VOCAB_ID}. A full VOCAB_URI can be used instead or, in the case of the object endpoint, uri={VOCAB_URI} and the vocab_id or vocab_uri parameter left out entirely in object

Still, yes, since the VOCAB_IDs are generated, the generation should be documented.