Expand comment explaining reasons for automatic swapoff on shutdown

Reviewed by:	alc, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D33167

sys/kern/vfs_bio.c

@@ -1458,9 +1458,16 @@ bufshutdown(int show_busybufs)
 			printf("Final sync complete\n");
 
 		/*
-		 * Unmount filesystems.  Swapoff before unmount,
-		 * because file-backed swap is non-operational after unmount
-		 * of the underlying filesystem.
+		 * Unmount filesystems and perform swapoff, to quiesce
+		 * the system as much as possible.  In particular, no
+		 * I/O should be initiated from top levels since it
+		 * might be abruptly terminated by reset, or otherwise
+		 * erronously handled because other parts of the
+		 * system are disabled.
+		 *
+		 * Swapoff before unmount, because file-backed swap is
+		 * non-operational after unmount of the underlying
+		 * filesystem.
 		 */
 		if (!KERNEL_PANICKED()) {
 			swapoff_all();