pgsql: Doc: improve documentation about nextval()/setval().

Doc: improve documentation about nextval()/setval().

Clarify that the results of nextval and setval are not guaranteed
persistent until the calling transaction commits.  Some people
seem to have drawn the opposite conclusion from the statement that
these functions are never rolled back, so re-word to avoid saying
it quite that way.

Discussion: https://postgr.es/m/CAKU4AWohO=NfM-4KiZWvdc+z3c1C9FrUBR6xnReFJ6sfy0i=Lw@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6365e3a0535fc5e6cd55ec4a97c562a11e371fdd

Modified Files
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doc/src/sgml/func.sgml | 61 +++++++++++++++++++++++++++++---------------------
1 file changed, 36 insertions(+), 25 deletions(-)