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Prompted in part by
https://github.com/ipld/go-ipld-prime/pull/21#issuecomment-518115245 .

Turns out some of the stuff our earliest Node implementations do are
actually pretty unusual.  Many other features have more strictures.
Therefore, it's important to specify the behavior such that the
features that have to operate within more restrictions can still
comply, and it's clear that anything else shouldn't be expected!

(Perhaps we should just... make the ipldfree.Node implementations not
do the flexible things they do right now.  That package is starting
to look in need of some renovation anyway.  We'll see.)
Signed-off-by: Eric Myhre <hash@exultant.us>