pgsql: Correct obsolete nbtree page deletion comment.

Correct obsolete nbtree page deletion comment.

Commit efada2b8e92, which made the nbtree page deletion algorithm more
robust, removed _bt_getstackbuf() calls from _bt_pagedel().  It failed
to update a comment that referenced the earlier approach.  Update the
comment to explain that the _bt_getstackbuf() page deletion call site
mirrors the only other remaining _bt_getstackbuf() call site, which is
reached during page splits.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/067786cea0f150ccf9350abb9a5ac3e4ad328804

Modified Files
--------------
src/backend/access/nbtree/nbtpage.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)