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_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/769eaef7600b0cc63bdc488428c4e01902261a73

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(-)