[1.1.X] Fixed #13317 - Clarified documentation about how the blocktrans and...

[1.1.X] Fixed #13317 - Clarified documentation about how the blocktrans and trans template tags work with regard to variables. Thanks for the initial patch, Ramiro Morales.

Backport from trunk, r13184.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@13185 bcc190cf-cafb-0310-a4f2-bffc1f526a37