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_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c909bd87ce09b00c15707c9420a01bfc8facad3f

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