DOC: correct merge_ordered example
- update of incorrect documentation example for merge_ordered.
- adding a test corresponding to this example.

- [ ] closes #19393 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @raph-m! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on January 25, 2018 at 22:50 Hours UTC Hi,
That's my first contribution to an open source project. Please let me know if I did something wrong :) Hello, the appveyor test did not pass but the errors do not appear to be related to my modifications. Please let me know if I should make changes to pass the AppVeyor test. @raph-m Don't worry about the appveyor tests, that is indeed unrelated (and being fixed)

Looks good for the rest!