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>
3 files changed