Add more charge docs

[david-pl]

@amilsted the build fails because of the example in the docstring. There's an UndefVarError somewhere.

Side note: I'll update the workflow to trigger on PRs without deployment automatically so we can detect these issues more easily and before merging (not that it matters much to have a failing build on master in this repo...)

[amilsted]

Thanks @david-pl I've fixed that (and one or two other things) on master.