JSONB operator clarification

In studying our JSONB operators, I was confused about what they do based
on the docs.  I found that "contain within" means "contain the
path/value", i.e. it has to match the path from the top level, not just
anywhere inside the document:

    SELECT '{"a":1, "c" : {"b":2}}'::jsonb @> '{"a":1}'::jsonb;
     ?column?
    ----------
     t

    SELECT '{"a":1, "c" : {"b":2}}'::jsonb @> '{"b":2}'::jsonb;
     ?column?
    ----------
     f

You can also specify only the top part of the path:

    SELECT '{"a":1, "c" : {"b":2}}'::jsonb @> '{"c":{}}'::jsonb;
     ?column?
    ----------
     t

    SELECT '{"a":1, "c" : {"b":2}}'::jsonb @> '{"d":{}}'::jsonb;
     ?column?
    ----------
     f

I also found that "key/element string exist" really means "string exist
as a top-level key", e.g.

    SELECT '{"a":1, "c" : {"b":2}}'::jsonb ? 'a';
     ?column?
    ----------
     t

    SELECT '{"a":1, "c" : {"b":2}}'::jsonb ? 'b';
     ?column?
    ----------
     f

The attached doc patch and SQL comment update improves this.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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