[1.3.X] Fixed #16005 -- Error in blocktrans docs -- thanks bezidejni

If you need to bind more than one expression inside a ``blocktrans`` tag,

separate the pieces with ``and``::

    {% blocktrans with book_t=book|title author_t=author|title %}

    {% blocktrans with book_t=book|title and author_t=author|title %}

    This is {{ book_t }} by {{ author_t }}

    {% endblocktrans %}