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>
diff --git a/Doc/glossary.rst b/Doc/glossary.rst
index 6189cb0..e997d36 100644
--- a/Doc/glossary.rst
+++ b/Doc/glossary.rst
@@ -189,6 +189,10 @@
       A list of bytecode instructions can be found in the documentation for
       :ref:`the dis module <bytecodes>`.
 
+   callback
+      A subroutine function which is passed as an argument to be executed at
+      some point in the future.
+
    class
       A template for creating user-defined objects. Class definitions
       normally contain method definitions which operate on instances of the