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_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6d1bdd5e0712c0c3460d68dc2175f842530b1f83

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