pgsql: Fix nbtree backward scan race condition comments.

Fix nbtree backward scan race condition comments.

Remove comments that supposed that holding a pin was a useful interlock
for _bt_walk_left().  There are times when _bt_walk_left() doesn't hold
either a lock or a pin on any page, so clearly this can't be true.
_bt_walk_left() is even prepared to deal with concurrent deletion of
both the original page and any pages to its left.

Oversight in commit 2ed5b87f96.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aa210e0c121eb8f58c86d4fcc833a5a6fbb6f5a9

Modified Files
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src/backend/access/nbtree/nbtsearch.c | 27 ++++++++-------------------
1 file changed, 8 insertions(+), 19 deletions(-)