pgsql: doc: mention GROUP BY columns can reference target col numbers

doc: mention GROUP BY columns can reference target col numbers

Reported-by: hape <postgres-hape@gmx.de>

Discussion: /messages/by-id/168871536004.379168.9352636188330923805@wrigleys.postgresql.org

Backpatch-through: 11

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3a9a1b821e07f1d223d2161e46286ee8d134163c

Modified Files
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doc/src/sgml/ref/select.sgml | 3 +++
1 file changed, 3 insertions(+)