Update parsers.py usecols description
Added note on the description of usecols in parsers.py file to clarify that order of parsers.py doesn't matter in the read_csv() method. Regardless of what order usecols is inputted in, read_csv() will return a DataFrame of the columns in ascending order.
Hello @jocelynshen! Thanks for submitting the PR.

 - In the file [`pandas/io/parsers.py`](https://github.com/pandas-dev/pandas/blob/a74f3930d9e8b78b722cba3feb9839ba342f04a5/pandas/io/parsers.py), following are the PEP8 issues :

> [Line 111:80](https://github.com/pandas-dev/pandas/blob/a74f3930d9e8b78b722cba3feb9839ba342f04a5/pandas/io/parsers.py#L111): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (87 > 79 characters)

 this duplicates #18729 (just a little bit before). thanks! would be happy to take a PR on other issues!