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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f3c2fae0636ed7e9e1a6a4365192ae8ef7af425f

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