Improve asyncio.loop.call_soon() documentation (GH-20883)

* Add a glossary entry for the term "callback"
* Link to it in loop.call_soon() and in the "Concurrency and Multithreading" section

Co-authored-by: Kyle Stanley <aeros167@gmail.com>
(cherry picked from commit a16d6970496cae5ecab5aaea2b416a4b77527cc2)

Co-authored-by: Roger Iyengar <ri@rogeriyengar.com>

2 files changed, 3 insertions, 3 deletions

diff --git a/Doc/library/asyncio-dev.rst b/Doc/library/asyncio-dev.rst
index 101e781..0d302ea 100644
--- a/Doc/library/asyncio-dev.rst
+++ b/Doc/library/asyncio-dev.rst
@@ -73,7 +73,7 @@ event loop, no other Tasks can run in the same thread.  When a Task
 executes an ``await`` expression, the running Task gets suspended, and
 the event loop executes the next Task.
 
-To schedule a callback from a different OS thread, the
+To schedule a :term:`callback` from another OS thread, the
 :meth:`loop.call_soon_threadsafe` method should be used. Example::
 
     loop.call_soon_threadsafe(callback, *args)
diff --git a/Doc/library/asyncio-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index 9022993..32bc219 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -179,8 +179,8 @@ Scheduling callbacks
 
 .. method:: loop.call_soon(callback, *args, context=None)
 
-   Schedule a *callback* to be called with *args* arguments at
-   the next iteration of the event loop.
+   Schedule the *callback* :term:`callback` to be called with
+   *args* arguments at the next iteration of the event loop.
 
    Callbacks are called in the order in which they are registered.
    Each callback will be called exactly once.