Add docstrings to functions in braginskii.py

[namurphy]

There are several functions in braginskii.py that require docstrings, including:

• resistivity
• thermoelectric_conductivity
• ion_thermal_conductivity
• electron_thermal_conductivity
• ion_viscosity
• electron_viscosity

This is different from but very related to #308, which requests docstring improvements for methods/attributes of the ClassicalTransport class.

[JvPy]

Would like to help with this issue.

Im not expert in the subject, but I can find references to the functions; also would like to make some contribution to documentation, if needed as well

[Tiger-Du]

I believe that this issue in its current scope is resolved now -- although we could expand its scope to include improving the docstrings. For example, formulas are missing for thermoelectric_conductivity, ion_viscosity, and electron_viscosity.

But perhaps it is best that we postpone improving the docstrings until we settle on a refactored version of braginskii.py as in PR #1121.

[namurphy]

Good points! I'm thinking that a lot of the discussions of the physics will end up being copied over from the original version to the refactored version, so adding formulae to the old versions would still be helpful. With that said, I agree that it makes more sense to figure out the refactoring in #1121 first.