Correct name for json_serializer / json_deserializer, document and test

Correct name for json_serializer / json_deserializer, document and test

The dialects that support json are supposed to take arguments
``json_serializer`` and ``json_deserializer`` at the create_engine() level,
however the SQLite dialect calls them ``_json_serilizer`` and
``_json_deserilalizer``.  The names have been corrected, the old names are
accepted with a change warning, and these parameters are now documented as
:paramref:`.create_engine.json_serializer` and
:paramref:`.create_engine.json_deserializer`.

Fixes: #4798
Change-Id: I1dbfe439b421fe9bb7ff3594ef455af8156f8851