Improve documentation for validate_holdout_datasets

[dsherry]

After #3208 the helper validate_holdout_datasets is only used in test_gen_utils.py. Let's delete it unless there's a justifiable reason to keep it. If we do keep it, let's improve the docstring since its currently vague: "Validate the holdout datasets match our expectations."

[freddyaboulton]

Our users use this function for time series problems. So let's not delete it and instead improve the docstring and maybe add it to the time series user guide. Renaming to reflect this.

[dsherry]

@freddyaboulton @ParthivNaresh 👍 SGTM!