be explicit in clear_status_flags_on_sybil that we leave BadExit alone

src/or/dirserv.c

@@ -2233,6 +2233,10 @@ dirserv_set_routerstatus_testing(routerstatus_t *rs)
 /** Routerstatus <b>rs</b> is part of a group of routers that are on
  * too narrow an IP-space. Clear out its flags: we don't want people
  * using it.
+ *
+ * Leave its BadExit flag alone though, since if we think it's a bad exit,
+ * we want to vote that way in case all the other authorities are voting
+ * Running and Exit.
  */
 static void
 clear_status_flags_on_sybil(routerstatus_t *rs)