[1.2.X] Fixed #10545 -- Mentioned that template context variables only exist...

[1.2.X] Fixed #10545 -- Mentioned that template context variables only exist in the scope of th block in which they're assigned. Thanks to yaniv.haber for the report and timo for the patch.

Backport of [14350] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14351 bcc190cf-cafb-0310-a4f2-bffc1f526a37