pgsql: Add note about CreateStatistics()'s selective use of check_right

Add note about CreateStatistics()'s selective use of check_rights.

Commit 5e4fcbe531 added a check_rights parameter to this function
for use by ALTER TABLE commands that re-create statistics objects.
However, we intentionally ignore check_rights when verifying
relation ownership because this function's lookup could return a
different answer than the caller's. This commit adds a note to
this effect so that we remember it down the road.

Reviewed-by: Noah Misch <noah(at)leadboat(dot)com>
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4c179ccb05062ebf29c2936e67842010d71742d0

Modified Files
--------------
src/backend/commands/statscmds.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)