Improve docstrings of magnetic field functions for prisms

[santisoler]

Page that contains the error or needs to be improved (paste link): https://www.fatiando.org/choclo/v0.2.0/api/generated/choclo.prism.magnetic_e.html#choclo.prism.magnetic_e

Description of the problem/suggestion:

I would remove the mathematical form of the kernels and point users to the docstrings of the actual kernel functions being used. Those simple mathematical forms are fine to have an understanding of the problem, but the kernels we use are more complex than that, since we are also considering limit cases and techniques to reduce floating point errors.