Problem: fix for read-after-free not documented

Solution: add a comment as it's easy to miss and very tempting to
"optimize"

src/radio.cpp

@@ -100,6 +100,8 @@ void zmq::radio_t::xwrite_activated (pipe_t *pipe_)
 
 void zmq::radio_t::xpipe_terminated (pipe_t *pipe_)
 {
+    //  NOTE: erase invalidates an iterator, and that's why it's not incrementing in post-loop
+    //  read-after-free caught by Valgrind, see https://github.com/zeromq/libzmq/pull/1771
     for (subscriptions_t::iterator it = subscriptions.begin (); it != subscriptions.end (); ) {
         if (it->second == pipe_) {
             subscriptions.erase (it++);