text: Title: Improvements to K-Elbow Visualizer
  Body: The following tasks from #91 are left over to finalize the implementation of the `KElbowVisualizer ` and improve it. 
  
  *Note to contributors: items in the below checklist don't need to be completed in a single PR; if you see one that catches your eye, feel to pick it off the list!*
  
  - [ ] Add inertia metric as seen in [sklearn KMeans Equivalent of Elbow Method](https://stackoverflow.com/questions/41540751/sklearn-kmeans-equivalent-of-elbow-method)
  - [x] Improve documentation with a description of how to use K-Elbow to select `k` 
  
  Stretch goals:
  
  - [x] Find example dataset with clear elbow curve demonstration to use in documentation 
  - [ ] Parallelize with joblib and `n_jobs` argument  (Note: this one is expert-level! See discussion [here](https://github.com/DistrictDataLabs/yellowbrick/pull/822#issuecomment-488258026) for more details)
  
  The stretch goals can be converted to their own issues if not completed in this feature request.