Add a note about 'NOT NULL' JSONB 'null'

```
create table a (
b jsonb not null default 'null'
);
```
We expose this as "non-nullable", but it's possible to store a JSON 'null' (rather than an SQL null) in the column. PostGraphile doesn't differentiate these, and trusts you'll do the right thing. Otherwise _all_ JSON/JSONB columns would have to be nullable, and that'd not be a pleasant experience.