[doc] bot owner can specify their own session object
[doc] bot owner can specify their own session object

- Add documentation how to use their own session object with Pywikibot.
- Make http.flush() public that it can be used from external
  (on their own risk)

Bug: T185561
Change-Id: I229a1c5e2262546221e54aed68942c7f23be07cc

wikimedia/pywikibot-core,wikimedia/pywikibot-core