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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master

Details
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https://git.postgresql.org/pg/commitdiff/4ac452e2285da347c75f5960ae211e183a87b57b

Modified Files
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doc/src/sgml/func.sgml | 31 ++++++++++++++++++++++---------
1 file changed, 22 insertions(+), 9 deletions(-)