pgsql: doc: improve ALTER SYSTEM description of value list quoting

doc: improve ALTER SYSTEM description of value list quoting

Reported-by: splarv@ya.ru

Discussion: https://postgr.es/m/167105927893.1897.13227723035830709578@wrigleys.postgresql.org

Backpatch-through: 11

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ff81b9b7a74e147b4da730d6023fa69f461903fc

Modified Files
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doc/src/sgml/ref/alter_system.sgml | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)