DOC/PLT: Add `stacked` in doc and doc example for barh and bar plot
- [x] closes #32759
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @charlesdong1991! 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 2020-05-05 14:36:22 UTC would appreciate a lot for some reviews, thanks! ^^ @jreback @WillAyd  Can you post a screenshot of the built docs? Doesn't this place the argument out of order (i.e. after the Returns section)?

@datapythonista  Hi, @WillAyd thanks for your review!

You are right, it is not the right place for parameters. And I removed it because i noticed another PR #33596 has included `stacked` also. So might be better to let that PR deal with parameter doc.

So the remaining question is the code example, I think it would be nice for users to see the example. But if you think it is not needed because its quite straightforward, then let's close this PR. I am open to hearing from your preference! ping @WillAyd 

pls let me know if it is still worth it to have this one in doc? ^^ Thanks @charlesdong1991 