pgsql: aio, bufmgr: Comment fixes/improvements

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/08ccd56ac765496a152ba50cf0ae743c39396f52

Modified Files
--------------
src/backend/postmaster/postmaster.c |  2 +-
src/backend/storage/buffer/bufmgr.c | 10 ++++------
src/include/storage/aio.h           |  2 +-
src/include/storage/aio_internal.h  | 22 +++++++++++++++++++++-
4 files changed, 27 insertions(+), 9 deletions(-)