aio, bufmgr: Comment fixes/improvements

Some of these comments have been wrong for a while (12f3867f55), some I
recently introduced (da7226993f, 55b454d0e1). 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

src/backend/postmaster/postmaster.c

@@ -4401,7 +4401,7 @@ maybe_adjust_io_workers(void)
 			++io_worker_count;
 		}
 		else
-			break;				/* XXX try again soon? */
+			break;				/* try again next time */
 	}
 
 	/* Too many running? */