Improve docstring explaining `job_kwargs`

[zm711]

As I mentioned in #2696,

I only recently realized that a job was actually just a process and so technically could set n_jobs =100 even if they had one core (and they weren't on Windows which has a hard limit of 61 processes). So it might be better for us to strategize the best way to explain n_jobs. We added in the fraction which is now clearer, but a better explanation of what negative values mean and n_jobs > cores means.

To be honest I would love if we could incorporate @h-mayorquin's tutorial diagrams somewhere in our docs moving forward that help explain aggregation, slice, and memory chunks.

[h-mayorquin]

My take would be to restrict n_jobs to be no more than the virtual cores.