DOC: tf2ss zpk2ss note controller canonical form

There is only one way to convert from state space form to transfer function, but there are multiple state space forms for a given transfer function.  (I'm trusting that the comment in tf2ss accurately describes the code.  This is what Matlab does too.)
Merge pull request #3228 from endolith/patch-6

DOC: tf2ss zpk2ss note controller canonical form