Update bidi.py

Author: abbrowne126

google/api_core/bidi.py

@@ -715,7 +715,11 @@ def start(self):
             _LOGGER.debug("Started helper thread %s", thread.name)
 
     def stop(self):
-        """Stop consuming the stream and shutdown the background thread."""
+        """Stop consuming the stream and shutdown the background thread.
+
+        WARNING: Cannot be called within `_thread_main`, since it is not
+        possible to join a thread to itself.
+        """
         with self._operational_lock:
             self._bidi_rpc.close()