DOC: Added notes section in dropna method
In case of specifying both the optional parameters how and thresh then thresh takes the precedence

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

![Screenshot from 2021-01-17 19-12-34](https://user-images.githubusercontent.com/51440666/104845272-7997ec80-58fa-11eb-9d42-5f2d212e440c.png)