DOC: Improve code example for DataFrame.join
The modified code example does not have unique values in the 'key' column in the calling DataFrame `df`. With unique keys the special behavior of `join` has not been highlighted, since it just added a new column. With the modified values the example shows that the 'key' column is really used as key.
OK, should I add it for all calls or just with the last and maybe most interesting call, i. e. `df.join(other.set_index('key'), on='key')`? I think the last one is enough Hello @ChristofKaufmann! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-12-04 00:58:08 UTC thanks @ChristofKaufmann  Thank you!