Fix margin() documentation to address issue #195

[murrayrm]

No description provided.

[coveralls]

Coverage remained the same at 77.882% when pulling 9fd6ae7 on murrayrm:fix_margins_doc into 601b581 on python-control:master.

[coveralls]

Coverage remained the same at 77.882% when pulling 9fd6ae7 on murrayrm:fix_margins_doc into 601b581 on python-control:master.

[coveralls]

Coverage remained the same at 77.882% when pulling 9fd6ae7 on murrayrm:fix_margins_doc into 601b581 on python-control:master.

[coveralls]

Coverage remained the same at 77.882% when pulling 9fd6ae7 on murrayrm:fix_margins_doc into 601b581 on python-control:master.

[coveralls]

Coverage increased (+0.5%) to 78.395% when pulling 1593391 on murrayrm:fix_margins_doc into 601b581 on python-control:master.

[coveralls]

Coverage remained the same at 77.882% when pulling 9fd6ae7 on murrayrm:fix_margins_doc into 601b581 on python-control:master.

[roryyorke]

Why not omit "crossover" altogether? The parenthetical remarks are helpful -- I always get these mixed up.

[repagh]

I must admit that the Mathworks explanation is nice here https://www.mathworks.com/help/control/ref/margin.html

I don't agree with the terminology as you now propose.

Gain crossover frequency is where gain is (crosses!) 0dB -- corresponds to gain margin
we can call it Wgco or Wpm

Phase crossover (frequency) is where phase crosses -180 deg -- corresponds to phase margin
We can call it Wpco or wg

Gain margin crossover is simply confusing

Maybe say:
wg: frequency at gain margin (phase crossover point, phase = -180 deg)
wp: frequency at phase margin (gain crossover point, gain = 0dB)

[repagh]

@murrayrm, I added a commit on this PR. I also edited comments for the related stability_margins function.