CDRIVER-4533 Document and test behavior of bson_new_from_json with multiple JSONs (#1310)

* bson_new_from_json documentation with multiple JSONs

* Tests

* Apply suggestions from code review

Co-authored-by: Kevin Albertson <kevin.albertson@10gen.com>
Co-authored-by: Ezra Chung <88335979+eramongodb@users.noreply.github.com>

* Mark current behavior as deprecated

* test simplifications

* Apply suggestions from code review

Co-authored-by: Kevin Albertson <kevin.albertson@10gen.com>

* Newline

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Co-authored-by: Kevin Albertson <kevin.albertson@10gen.com>
Co-authored-by: Ezra Chung <88335979+eramongodb@users.noreply.github.com>