Update some comments that should've covered MERGE

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: fb2a83b2b750a32ddfd107a75a3bc173f4f0a81f
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2022-10-24T10:52:43Z
Releases: 15.1
Update some comments that should've covered MERGE

Oversight in 7103ebb7aae8.  Backpatch to 15.

Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAMbWs48gnDjZXq3-b56dVpQCNUJ5hD9kdtWN4QFwKCEapspNsA@mail.gmail.com

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