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_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a6088e1f5a58e47ee6f207ec13c67773852df970

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(-)