Fixes #2384,#2566 -- Clarify the role that TEMPLATE_STRING_IF_INVALID plays in the template system, and the problems that can occur if it is used on a production site. 


git-svn-id: http://code.djangoproject.com/svn/django/trunk@3901 bcc190cf-cafb-0310-a4f2-bffc1f526a37