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_2_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/80af818dd618b18d8e4b3f629844190473f2bae2

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(-)