bpo-31960: Fix asyncio.Future documentation for thread (un)safety. (GH-4319) (#4320)

(cherry picked from commit 22b11285)

Doc/library/asyncio-task.rst

@@ -216,7 +216,7 @@ Future
      raise an exception when the future isn't done yet.
 
    - Callbacks registered with :meth:`add_done_callback` are always called
-     via the event loop's :meth:`~AbstractEventLoop.call_soon_threadsafe`.
+     via the event loop's :meth:`~AbstractEventLoop.call_soon`.
 
    - This class is not compatible with the :func:`~concurrent.futures.wait` and
      :func:`~concurrent.futures.as_completed` functions in the

Lib/asyncio/futures.py

@@ -112,11 +112,13 @@ class Future:
 
     Differences:
 
+    - This class is not thread-safe.
+
     - result() and exception() do not take a timeout argument and
       raise an exception when the future isn't done yet.
 
     - Callbacks registered with add_done_callback() are always called
-      via the event loop's call_soon_threadsafe().
+      via the event loop's call_soon().
 
     - This class is not compatible with the wait() and as_completed()
       methods in the concurrent.futures package.