document why we only allow 64 flags in votes

src/or/routerparse.c

@@ -3006,6 +3006,11 @@ networkstatus_parse_vote_from_string(const char *s, const char **eos_out,
   }
   if (ns->type != NS_TYPE_CONSENSUS &&
       smartlist_len(ns->known_flags) > MAX_KNOWN_FLAGS_IN_VOTE) {
+    /* If we allowed more than 64 flags in votes, then parsing them would make
+     * us invoke undefined behavior whenever we used 1<<flagnum to do a
+     * bit-shift. This is only for votes and opinions: consensus users don't
+     * care about flags they don't recognize, and so don't build a bitfield
+     * for them. */
     log_warn(LD_DIR, "Too many known-flags in consensus vote or opinion");
     goto err;
   }