A docstring would be better than the tutorial I think, easier to find and the link is much shorter.

``lti`` doesn't have an explanation yet, and the naming doesn't match (``b,a`` vs. ``num,den``). The Notes section of the ``lti`` docstring does look like a good place to explain all 3 representations though. The transfer function representation can then be taken over from ``freqs``, because that does have it already.