I have come across some missing documentation that I think could benefit the community.
Several functions like `jsonb_exists`, `jsonb_exists_any`, `jsonb_exists_all` have existed for many PG versions but were not documented. They are equivalent to `?`, `?|`, and `?&` operators. But some JDBC drivers have issues with native queries containing these operators (see https://stackoverflow.com/questions/38370972/how-do-i-use-postgresql-jsonb-operators-containing-a-question-mark-via-jdb), so it is useful for users of PG to know the function equivalents of these operators.
I have attached the patch as an attachment to this email. The documentation builds correctly without any lint errors after applying the patch locally. This is my first time contributing, so let me know if there is anything else I should do (add to commitfest etc).
