Added warning about replay attacks when using the cookies backend for sessions.
Added warning about replay attacks when using the cookies backend for sessions.

The paragraph about encryption was reworded for clarity.

git-svn-id: 554f83ef17aa7291f84efa897c1acfc5d0035373@17004 bcc190cf-cafb-0310-a4f2-bffc1f526a37

svn2github/django,svn2github/django,svn2github/django