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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master

Details
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https://git.postgresql.org/pg/commitdiff/ed3dc224e5aabd3cb0a5c348107f973fe5b10b0d

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(-)