pgsql: Doc: improve documentation for UNNEST().

Doc: improve documentation for UNNEST().

Per a user question, spell out that UNNEST() returns array elements
in storage order; also provide an example to clarify the behavior for
multi-dimensional arrays.

While here, also clarify the SELECT reference page's description of
WITH ORDINALITY.  These details were already given in 7.2.1.4, but
a reference page should not omit details.

Back-patch to v13; there's not room in the table in older versions.

Discussion: https://postgr.es/m/FF1FB31F-0507-4F18-9559-2DE6E07E3B43@gmail.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bfda0a02444e204024d83db2874ec884960d24f0

Modified Files
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doc/src/sgml/func.sgml       | 19 +++++++++++++++----
doc/src/sgml/ref/select.sgml | 17 ++++++++++++-----
2 files changed, 27 insertions(+), 9 deletions(-)