DOC: DataFrame.reindex improve summary and return type
- [x] closes #33888 
- [x] tests added / passed (passes `scripts/validate_docstrings.py pandas.DataFrame.reindex`)
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry (Not Required)
Hello @gauravchandok! 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 2020-06-06 21:18:56 UTC While I'm reading your changes I realized there is more to this confusion than just the docstring. See #34663. The behavior of `copy` is confusing in my opinion. We probably need to address that first. 

At the moment, I don't think this PR explains its behavior accurately. Let's discuss in #34663 what the desired outcome should be, which could be to deprecate `copy`. After that you can continue working on this PR or open a new PR to achieve that. @gauravchandok Can you merge master to fix conflicts? This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. @gauravchandok Can you address comments and merge master? Closing for now. @gauravchandok ping us whenever you'd like to continue and we'll reopen!