rootlocus_pid_designer plot looks strange + needs documentation

From PR #684:

If I use the following code (from the now skipped unit test in control/tests/rootlocus_test.py:

syscont = ct.TransferFunction(1,[1, 3, 0])
ct.rootlocus_pid_designer(syscont, input_signal='r', Kp0=0.01,
    derivative_in_feedback_path=True)

then I get the following plot: