Move citations of unpublished reference from docstrings to comments for formulary.collisions.frequencies #3192
Description
The docstrings in plasmapy.formulary.collisions.frequencies frequently cite Draft Material For "Fundamentals of Plasma Physics" Book, which is published on google docs. While this draft book is a valuable resource that thoroughly explains much of what the module covers, there is a non-negligible chance of the hyperlink breaking in the years to come. Alternatively, the chapters could get updated over time and the equation numbers could change.
It would be helpful to move the references to these draft materials from docstrings to comments so that it becomes less likely that a whole bunch of links in the online documentation all break at once.
Alternatively, if this reference gets openly and permanently archived on a site like Zenodo 🎉, we should update the bibliography entry to include a DOI and keep citing it.
I'm going to download all of the chapters so that I have a copy of them handy in case the link breaks.