update ellipsoid documentation

[lucas-wilkins]

As originally noted in SasView/sasview#2508, the ellipsoid model documentation needs some work. Besides a review and better description of the model, including the difference between oblate and prolate ellipsoids and the need to start with the parameters the right way around to avoid local minima, a graphic should be provide which was provided by Andreas in SasView/sasview#2508. Also the validation section probably should be removed since it does not match the rest of the docs and does not do anything for "validation" as it is anyway.