Fix bogus comments

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

Commit: cca2a278609e9ed9bedc4796ae1427fbfb88a2fc
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2016-05-12T19:07:07Z
Releases: 9.6.0
Fix bogus comments

Some comments mentioned XLogReplayBuffer, but there's no such function:
that was an interim name for a function that got renamed to
XLogReadBufferForRedo, before commit 2c03216d831160 was pushed.

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