Fix incorrect comment about the timing of AbsorbFsyncRequests() during

Fix incorrect comment about the timing of AbsorbFsyncRequests() during
checkpoint.  The comment claimed that we could do this anytime after
setting the checkpoint REDO point, but actually BufferSync is relying
on the assumption that buffers dumped by other backends will be fsync'd
too.  So we really could not do it any sooner than we are doing it.