[1.2.X] Fixed #13754 - Add a note about a test client session property gotcha

  
Thanks SmileyChris for report and patch.

Backport of [13685] from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13686 bcc190cf-cafb-0310-a4f2-bffc1f526a37