JSONB docs patch

Hello,

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).

Cheers!
Mikhail Dobrinin