aio, bufmgr: Comment fixes/improvements
Some of these comments have been wrong for a while (
12f3867f5534), some I
recently introduced (
da7226993fd,
55b454d0e14). This includes an update to a
comment in FlushBuffer(), which will be copied in a future commit.
These changes seem big enough to be worth doing in separate commits.
Suggested-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/
20250319212530.80.nmisch@google.com