text: Title: underperforming group 'about this issue' print format
  Body: Fix this format:
  
  ![Screen Shot 2024-06-18 at 9 39 53 PM](https://github.com/cleanlab/cleanlab/assets/1390638/bcb60116-8639-42c2-93b0-245eadd69241)
  
  in fact, this statement is not very helpful to users (leads them down a rabbit hole of having to learn a lot):
  
  > An underperforming group refers to a collection of “hard” examples
      for which the model predictions are poor.  The quality of predictions is computed using the :py:func:`get_self_confidence_for_each_label <cleanlab.rank.get_self_confidence_for_each_label>` function.
  
  Instead just say:
  
  > An underperforming group refers to a cluster of similar examples (i.e. a slice) in the dataset for which the ML model predictions are particularly poor (loss evaluation over this subpopulation is high).
  
  