pgsql: Doc: mention that extended stats aren't used for joins

Doc: mention that extended stats aren't used for joins

Statistics defined by the CREATE STATISTICS command are only used to
assist with the selectivity estimations of base relations, never for
joins.  Here we mention this fact in the notes section of the CREATE
STATISTICS command.

Discussion: https://postgr.es/m/CAApHDvrMuVgDOrmg_EtFDZ=AOovq6EsJNnHH1ddyZ8EqL4yzMw@mail.gmail.com
Backpatch-through: 11

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4e17cb00dc2b57f0ac1bbe3c1fd587e982b33307

Modified Files
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doc/src/sgml/ref/create_statistics.sgml | 6 ++++++
1 file changed, 6 insertions(+)