Centralize non-JSON considerations

[cabo]

Centralize the text that explains how non-interoperable JSON may cause unpredictable behavior.

This is a little clumsy as the sections to be referenced are currently unnumbered.

[cabo]

Solves a few "behaviours", too :-)

[goessner]

This centralizing approach is a good thing.

I suggest to reuse the phrase "duplicate names" for better recognition of the wording in RFC8259, though

multiple members for a single object that share the same member name

is more general, as it addresses more than two.