Fix comments in gistxlogDelete, xl_heap_freeze_page and xl_btree_delete

Hi hackers,

Please find attached a patch to $SUBJECT.

The wrong comments have been discovered by Robert in [1].

Submitting this here as a separate thread so it does not get lost in the logical decoding
on standby thread.

[1]: https://www.postgresql.org/message-id/CA%2BTgmoYTTsxP8y6uknZvCBNCRq%2B1FJ4zGbX8Px1TGW459fGsaQ%40mail.gmail.com

Looking forward to your feedback,

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com