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_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/36b607c9a8edfa47770be3a18fe2c9215595decb

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