[Issue: #16416] Adding examples for some DataFrame methods
Adding examples for [`fillna`](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.fillna.html), [`drop`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.drop.html), and [`sort_values`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.sort_values.html).
 - [x] related to #16416 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry
