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_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6d0dc1a7da7942915c8160caa379c58dfd1488b8

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