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)
Hello @shubhamsharma10! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/core/frame.py`](https://github.com/pandas-dev/pandas/blob/a7013b6977e7a7eccb84bb12ebcd467934c01aaa/pandas/core/frame.py):

> [Line 5160:89](https://github.com/pandas-dev/pandas/blob/a7013b6977e7a7eccb84bb12ebcd467934c01aaa/pandas/core/frame.py#L5160): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (90 > 88 characters)

