docs: explain default session expiration (#36126)

Author: robrap

openedx/core/djangoapps/session_inactivity_timeout/middleware.py

@@ -7,6 +7,9 @@
   SESSION_INACTIVITY_TIMEOUT_IN_SECS = 300
 
 This was taken from StackOverflow (http://stackoverflow.com/questions/14830669/how-to-expire-django-session-in-5minutes)
+
+If left unset, session expiration will be handled by Django's SESSION_COOKIE_AGE,
+which defauts to 1209600 (2 weeks, in seconds).
 """