#2469: None of the thorns in WVUThorns_Diagnostics have documentation

Comment (by Zach Etienne):

Thanks for the reminder @Roland Haas . I’ve asked Leo if he might be able to document at least one more thorn for the Nov 2021 ET release.

Regarding smallbPoynET: Note that we have an updated version of this thorn, which is fully documented here (main github: https://github.com/zachetienne/nrpytutorial):
https://nbviewer.org/github/zachetienne/nrpytutorial/blob/master/Tutorial-ETK_thorn-u0_smallb_Poynting.ipynb
I am not proposing inclusion of this thorn (major update to existing thorn) into the Nov 2021 ET release.

The more appropriate Jupyter notebook documenting the current thorn would be this one, which reviews the basic equations that are implemented:
https://nbviewer.org/github/zachetienne/nrpytutorial/blob/master/Tutorial-u0_smallb_Poynting-Cartesian.ipynb