Fixing 'names' doc for 'read_csv'
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I think there is a bug in the docs. This Jupyter notebook shows that the code does not work as expected from the docs https://gist.github.com/bessarabov/5e42bbb359fb86aaa87aed6c12a95bc9

So I've fixed the doc to match behaviour.

I made one more python notebook to check how the code works when there is no header in csv:

 * [pandas read_csv no-header.ipynb](https://gist.github.com/bessarabov/ff2ba9920e6b6823112611e7c3604289)
 * [pandas read_csv with-header.ipynb](https://gist.github.com/bessarabov/e102f8a29108d08b7da1617cf09fe38d)

The current doc text is:

> names : array-like, optional
> List of column names to use. If file contains no header row, then you should explicitly pass
> header=None. Duplicates in this list are not allowed.

As you can see from [pandas read_csv no-header.ipynb](https://gist.github.com/bessarabov/ff2ba9920e6b6823112611e7c3604289)
 * [pandas read_csv with-header.ipynb] there is no need to explicitly pass `header=None` the code works without `header=None` as it does with `header=None`

So I think there should be no such text in the doc.

But when there is a header in csv file you must specify `header=0` to make the code work as expected — [pandas read_csv with-header.ipynb](https://gist.github.com/bessarabov/e102f8a29108d08b7da1617cf09fe38d) thanks @bessarabov 