Issue #18277: Document quirks of multiprocessing queue.
diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 6f9298a..8904cc3 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -486,6 +486,23 @@
the :mod:`multiprocessing` namespace so you need to import them from
:mod:`Queue`.
+.. note::
+
+ When an object is put on a queue, the object is pickled and a
+ background thread later flushes the pickled data to an underlying
+ pipe. This has some consequences which are a little surprising,
+ but should not cause any pratical difficulties -- you can always
+ use a managed queue if they really bother you.
+
+ (1) After putting an object on an empty queue there may be an
+ infinitessimal delay before the queue's :meth:`~Queue.empty`
+ method returns :const:`False` and :meth:`~Queue.get_nowait` can
+ return without raising :exc:`Queue.Empty`.
+
+ (2) If multiple processes are enqueuing objects, it is possible for
+ the objects to be received at the other end out-of-order.
+ However, objects enqueued by the same process will always be in
+ the expected order with respect to each other.
.. warning::