Fix JsonGenerator.close regression

Please check https://github.com/jakartaee/jsonp-api/issues/156, this issue introduced by a too quick review a big regression on JSON-P API design (and implementations/doc bugs) so it is more than likely and welcomed to revert this by documenting properly the behavior and harnessing it instead of breaking the JSON-P api design.

Start by reading the linked JSON-P API issue 156 and the discussion in this issue to establish the intended JsonGenerator.close behavior. Identify the affected API, implementation, and documentation areas, then define completion as restoring the intended design with documented behavior and a harness that verifies it.

Written by the indexing model from the issue text.