[docs] [issue15339] document the threading "facts of life" in Python

Change by Irit Katriel : ---------- priority: low -> normal stage: -> needs patch type: -> behavior versions: +Python 3.10, Python 3.9 -Python 2.7, Python 3.2, Python 3.3 _______________________________________ Python tracker https://bugs.python.org/issue15339 _______________________________________