document gevent monkey patching side effect

when terminating gevent's hub and waiting for remaining greenlets

Author: ultrabug

doc/examples/gevent.rst

@@ -16,3 +16,15 @@ PyMongo uses thread and socket functions from the Python standard library.
 Gevent's monkey-patching replaces those standard functions so that PyMongo
 does asynchronous I/O with non-blocking sockets, and schedules operations
 on greenlets instead of threads.
+
+Consideration on gevent's hub termination
+-----------------------------------------
+When shutting down your application gracefully, your code be may waiting
+for any remaining greenlet to terminate before exiting.
+Because monkey-patched threads are greenlets and PyMongo uses quite a lot
+of them, this may become a **blocking operation** and may prevent your
+application to exit properly. You therefore must close (or dereference)
+any active MongoClient object before exiting !
+
+**uWSGI** applications using the ``gevent-wait-for-hub`` option are directly
+affected by this behaviour.