DOC: add note about different ba forms in tf2zpk

of course someone should confirm that what I wrote is actually true...  

Feel free to suggest different wordings.

For comparison, matlab has 2 different functions: [`tf2zp`](http://www.mathworks.com/help/signal/ref/tf2zp.html) for s and positive z, and [`tf2zpk`](http://www.mathworks.com/help/signal/ref/tf2zpk.html) for z^-1.  Matlab also has the [`tf`](http://www.mathworks.com/help/control/ref/tf.html) function, which has a `{'s', 'z', 'z^-1'}` parameter.  A parameter like that should probably be added to scipy's `tf2zpk` function?  I think that's more usable than creating a separate `tf2zp` function (do mostly the same thing, hard to remember which is which).  But for now I am just documenting the discrepancy.