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_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b0a7161c564089d4119bbcd5e874a1f4430de1bc

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