More explicitly warn about pre_dispatch = 2*n_jobs with joblib.Parallel

[jacksonllee]

joblib.Parallel has pre_dispatch default to 2 * n_jobs, which seems unintuitive for a Civis Platform user expecting only n_jobs parallel jobs. The Sphinx docs do mention this behavior and subtly recommend setting n_jobs explicitly:

The default value of pre_dispatch is "2*n_jobs", which will run a maximum of 2 * n_jobs jobs at once in the Civis Platform. Set pre_dispatch="n_jobs" in your :class:~joblib.Parallel call to run at most n_jobs jobs.

Perhaps we should more explicitly warn about this in the documentation, with one of the Sphinx call-out note boxes. In this case, the .. warning:: paragraph directive would seem appropriate.