DOC: More subtotals / margins in pivot_table
- [ ] closes #4817 
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Added an example in the documentation for DataFrame.pivot_table to show how to display a pivot table as having a multi-level index, simulating Excel's pivot table with MI and partial group aggregates.