Fixed firstof docs error introduced in 1ea44a; refs #17906.

This is equivalent to::

    {% if var1 %}

        {{ var1|safe }}

        {{ var1 }}

    {% elif var2 %}

        {{ var2|safe }}

        {{ var2 }}

    {% elif var3 %}

        {{ var3|safe }}

        {{ var3 }}

    {% endif %}

You can also use a literal string as a fallback value in case all