Clearer docs for the quantities computed by the dielectric modules

Based on the discussion in #142 (closed), we should revise the documentation of the dielectric modules. For example, the dielectric cylinder states

Components are calculated along the axial (:math:z) and radial (:math:r) direction either with respect to the center of the simulation box or the center of mass of the refgroup, if provided. The axial direction is selected using the dim parameter.

One might be confused and could imagine that the module computes scalar profiles in z and in r, while we compute the dielectric tensor in the radial direction and we report its two components.

I think the same holds for the planar and the spherical module.