adding session engine section to sysadmin docs
giving more links to the Django doc
replacing prerequisites by the link to Django doc
links distinction
fixing spelling mistakes
adding Django clearing-the-session-store link
removing space
adding SESSION_EXPIRE_AT_BROWSER_CLOSE and SESSION_COOKIE_AGE
adding clearsessions to the windows doc
Merge pull request #834 from aleksandra-tarkowska/session_engine

Session engine