Fixed #17618 - Documented that variable names in template must not start with...

Fixed #17618 - Documented that variable names in template must not start with an underscore; thanks guillemette and krzysiumed.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17504 bcc190cf-cafb-0310-a4f2-bffc1f526a37