asyncio doc: list limitations to run subprocesses from different threads

Doc/library/asyncio-dev.rst

@@ -74,6 +74,11 @@ the event loop.
    The :ref:`Synchronization primitives <asyncio-sync>` section describes ways
    to synchronize tasks.
 
+   The :ref:`Subprocess and threads <asyncio-subprocess-threads>` section lists
+   asyncio limitations to run subprocesses from different threads.
+
+
+
 
 .. _asyncio-handle-blocking:
 

Doc/library/asyncio-subprocess.rst

@@ -297,6 +297,25 @@ Process
       ``N`` (Unix only).
 
 
+.. _asyncio-subprocess-threads:
+
+Subprocess and threads
+======================
+
+asyncio supports running subprocesses from different threads, but there
+are limits:
+
+* An event loop must run in the main thread
+* The child watcher must be instantiated in the main thread, before executing
+  subprocesses from other threads. Call the :func:`get_child_watcher`
+  function in the main thread to instantiate the child watcher.
+
+.. seealso::
+
+   The :ref:`Concurrency and multithreading in asyncio
+   <asyncio-multithreading>` section.
+
+
 Subprocess examples
 ===================