PlasmaPy
Directly use `valid_categories` to list particle categories in `is_category` method
Right now the list of particle categories (contained in particle_class.valid_categories) is typed out directly in the docstring for AbstractPhysicalParticle.is_category. The disadvantage of this is that if valid_particles changes, there's a chance that the docstring will become out-of-date. It would be helpful to use the contents of valid_particles to print out the list of categories.
This could possibly be done as an f-string, possibly after creating a separate function outside of the docstring). The formatting should say the same (i.e. double back ticks and double quotes around the categories, with lines limited to ≲72 characters) Since valid_categories is a set, the contents will need to be sorted alphabetically. This could possibly go in the docstring for valid_categories too.
This should be done after both #1720 and #1721 are merged.
