pgsql: doc: Mention CASCADE/RESTRICT for DROP STATISTICS

doc: Mention CASCADE/RESTRICT for DROP STATISTICS

This grammar has no effect as there are no dependencies on statistics,
but it is supported by the parser.  This is more consistent with the
other DROP commands.

Author: Vignesh C
Discussion: https://postgr.es/m/CALDaNm1LA=yNmzcSfy+0oe6CEAgsxXRf_-UutE3ZncFi8QkFNQ@mail.gmail.com
Backpatch-through: 10

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e159368027e342468bb7f9db0d8d42a76dc42f23

Modified Files
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doc/src/sgml/ref/drop_statistics.sgml | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)