Docs: clarify definition of pE vs. redox potential in documentation

[rkingsbury]

The Solution.__init__ documentation contains a brief description of the pE keyword argument, but does not specify how to convert between redox potential (Eh) and pE. Since redox potential is commonly used in electrochemistry and pE is somewhat domain-specific to geochemistry, the docs should explain how to convert between Eh and pE.

This page is a good resource for the necessary info. https://vulavav.people.charleston.edu/geochemistry/53pe.html