Mention threadpool interface in asyncio overview.
diff --git a/Doc/library/asyncio.rst b/Doc/library/asyncio.rst
index 914e381..f093e9e 100644
--- a/Doc/library/asyncio.rst
+++ b/Doc/library/asyncio.rst
@@ -33,6 +33,10 @@
* :ref:`synchronization primitives <sync>` for use between coroutines in
a single thread, mimicking those in the :mod:`threading` module;
+* an interface for passing work off to a threadpool, for times when
+ you absolutely, positively have to use a library that makes blocking
+ I/O calls.
+
Disclaimer
----------