Either document that `/metadata/json:submit` can be used to _update_ documents or prohibit that usage

[eecavanna]

Today, I learned that the POST /metadata/json:submit endpoint could be used to update documents (by their id), not just insert documents. Given that the endpoint lacks documentation, I didn't learn this until I read this comment and then tried it out locally (I was able to update an existing document).

To do:

• Determine whether the ability to update documents via that endpoint is necessary (prohibiting updates will simplify development). Ask stakeholders (e.g. @aclum).
• If it is necessary: The current referential integrity check may be inadequate (the referential integrity check was designed under the incorrect assumption that the endpoint could only be used to insert documents). In this case, revisit the referential integrity check.
• If it is necessary: Document it (i.e. updating documents) as an option, in the endpoint's documentation that appears on Swagger UI.
• If it is not necessary: Prohibit it and simplify the code.

[aclum]

I knew this was an option but I'm not aware of any uses of this in what I'd call production code since this is also more complicated as a developer would have to dump the current document, update it, and then re-add it. cc @sujaypatil96 please correct me if I'm wrong.

I suggest adding this as an agenda item for the infrastructure meeting on Thursday and disallowing this if there are no current dependencies on this functionality.