[Cohort Update]: Clarity in cohorting feature for observations

[kenviku]

It is not intuitive in the reporting function bias_report() that parameter cohorts applies only to the target variable or the y-label. Update the parameter to ensure that usage is clear for users that cohorting does not refer to any grouping on feature columns!

[camagallen]

@kenviku: We do already have examples in both the Binary Classification Tutorial and the Regression Tutorial, and at least one of these examples will be added to the READMEs. Docstrings of functions for which this will be enabled will also be updated with a brief description before the V2 release. Is this enough? Can you recommend any other updates to make this more clear?