Update a comment in brelse() to match reality.

sys/kern/vfs_bio.c

@@ -2340,9 +2340,18 @@ brelse(struct buf *bp)
 	    !(bp->b_flags & B_INVAL)) {
 		/*
 		 * Failed write, redirty.  All errors except ENXIO (which
-		 * means the device is gone) are expected to be potentially
-		 * transient - underlying media might work if tried again
-		 * after EIO, and memory might be available after an ENOMEM.
+		 * means the device is gone) are treated as being
+		 * transient.
+		 *
+		 * XXX Treating EIO as transient is not correct; the
+		 * contract with the local storage device drivers is that
+		 * they will only return EIO once the I/O is no longer
+		 * retriable.  Network I/O also respects this through the
+		 * guarantees of TCP and/or the internal retries of NFS.
+		 * ENOMEM might be transient, but we also have no way of
+		 * knowing when its ok to retry/reschedule.  In general,
+		 * this entire case should be made obsolete through better
+		 * error handling/recovery and resource scheduling.
 		 *
 		 * Do this also for buffers that failed with ENXIO, but have
 		 * non-empty dependencies - the soft updates code might need