[1.3.X] Fixed #17618 - Documented that variable names in template must not...

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

Backport of r17504 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@17505 bcc190cf-cafb-0310-a4f2-bffc1f526a37