pgsql: doc: split out the NATURAL/CROSS JOIN in SELECT syntax

doc: split out the NATURAL/CROSS JOIN in SELECT syntax

This allows the syntax to be more accurate about what clauses are
supported. Also switch an example query to use the ANSI join syntax.

Reported-by: Joel Jacobson

Discussion: /messages/by-id/67b71d3e-0c22-44df-a223-351f14418319@www.fastmail.com

Backpatch-through: 11

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/181cc0906b9130f0acb37131a50c06a601ec115c

Modified Files
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doc/src/sgml/ref/select.sgml | 46 +++++++++++++++++++++++---------------------
1 file changed, 24 insertions(+), 22 deletions(-)