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

doc:  mention GROUP BY columns can reference target col numbers

Reported-by: hape 

Discussion: https://postgr.es/m/168871536004.379168.9352636188330923805@wrigleys.postgresql.org

Backpatch-through: 11

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/28919a063d8c8b81ca574ac063e26304f41ed2fe

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