Hello,

This is a follow-up of #1004, still on pole placement, but now turning to acker docstring.

As a first step, this adds a "See Also" section to place and place varga.

However, I have two more ideas to extend this PR:

• add reference to the Ackermann's formula. I've found § 2.3.5 in the "Linear Control Theory" book by F. W. Fairman, 1998, but I'm not sure it matches perfectly with what's implemented.
• to the best of my (quite limited) knownledge, Ackermann's formula is for single input systems (and I got some linear algebra error on a test with a system with 2 inputs). If correct, I suggest to add a ValueError if B.shape[1] > 1.