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_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8145b5e138c7f26594fb93e11e6ebd5bf2aba586

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