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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master

Details
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https://git.postgresql.org/pg/commitdiff/c2d35bb88eab99fd81198fa4caee004fb8997c73

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