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: /messages/by-id/CALDaNm1LA=yNmzcSfy+0oe6CEAgsxXRf_-UutE3ZncFi8QkFNQ@mail.gmail.com
Backpatch-through: 10

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d1216635610a9855a8ec139e55cd852f67d5ee81

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