Add transport_model options to python module documentation

[d-e-t]

Problem description

The python module documentation for the transport_model method lacks reference to the model options. This documentation would be improved by adding the transport model labels, especially with the changes in label from Cantera 2.6 to 3.0. (for example from "Multi" to "multicomponent"). The Matlab documentation includes transport model options.

Cantera versions

Applies to documentation for the current stable 3.0.0 and development 3.1

[ischoegl]

@d-e-t ... thank you for your suggestion. Adding documentation is relatively straight-forward - in case you should be sufficiently familiar, feel free to start a PR to address this issue.

[d-e-t]

@ischoegl - Yes, this is pretty minor, and I think it is clearer in my edit.

[d-e-t]

Approved by @decaluwe -- thanks! Closing this now.