Document the specification/encoding of JSON accepted by the Data Service

What's the issue?
From my understanding we don't currently document which JSON specification/encoding is accepted by the Data Service. Considering there are multiple specifications it seems sensible to publicly document which are supported (closely tied to this is the JSON encoding i.e. UTF-8/16/32).

Note that in theory updated specifications obsolete old ones, however, that doesn't solve the fact that languages/implementations may/will still follow older specifications and additional checks may be required to be put in place to ensure the expected behavior.