Updated the docstring for reset_index in frame.py
If not all levels are removed, the index is not reset to default integer values.

import pandas as pd

x = pd.Series(range(3), index=pd.MultiIndex.from_arrays([['a', 'b', 'c'], [1, 2, 3]], names=['letters', 'numbers']))

print(x.reset_index('numbers', drop=True))

- [ ] closes #xxxx
- [ ] 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
