DOC: config: clarify some known limitations of the json_query() converter

DOC: config: clarify some known limitations of the json_query() converter

Oula Kivalo reported that different JSON libraries may process duplicate
keys differently and that most JSON libraries usually decode the stream
before extracting keys, while the current mjson implementation decodes the
contents during extraction instead. Let's document this point so that
users are aware of the limitations and do not rely on the current behavior
and do not use it for what it's not made for (e.g. content sanitization).

This is also the case for jwt_header_query(), jwt_payload_query() and
jwt_verify(), which already refer to this converter for specificities.