docs: add doc for json columns

[pYassine]

Hi !
I recently encountered a problem updating a column in JSON format. Here is a small contribution to indicate it directly in the documentation

[jhonnyizidoro]

Thank you @pYassine! You already helped me even before the merge

[porsager]

sql.json() is a remnant from v1, and not needed anymore, hence not in the docs.

There is an issue if your json value is not an object / array, but for that we have #392 in the works.