pgsql: Doc: clarify description of degenerate NATURAL joins.

Doc: clarify description of degenerate NATURAL joins.

Claiming that NATURAL JOIN is equivalent to CROSS JOIN when there are
no common column names is only strictly correct if it's an inner join;
you can't say e.g. CROSS LEFT JOIN. Better to explain it as meaning
JOIN ON TRUE, instead. Per a suggestion from David Johnston.

Discussion: https://postgr.es/m/CAKFQuwb+mYszQhDS9f_dqRrk1=Pe-S6D=XMkAXcDf4ykKPmgKQ@mail.gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5512427b19e79ac95535e7c942bd479db807f7be

Modified Files
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doc/src/sgml/queries.sgml | 4 ++--
doc/src/sgml/ref/select.sgml | 7 +++++--
2 files changed, 7 insertions(+), 4 deletions(-)