DOC: #22899, Fixed docstring of itertuples in pandas/core/frame.py
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

#22899 
I went ahead and followed the docstring conventions mentioned here: 

https://pandas.pydata.org/pandas-docs/stable/contributing_docstring.html

The fixes I made were for the following errors:

<img width="891" alt="screen shot 2018-09-29 at 19 17 59" src="https://user-images.githubusercontent.com/16315857/46252338-740df400-c41c-11e8-9c29-ff955c9b9705.png">
<img width="618" alt="screen shot 2018-09-29 at 19 18 08" src="https://user-images.githubusercontent.com/16315857/46252339-76704e00-c41c-11e8-89d5-adab168dc329.png">

- Fixed whitespace between closing line and end of docstring
- Converted summary into single line
- Added `Returns` section (Not sure if done correctly!)
- Fixed examples to pass test by adding `...` at line 914
- Fixed namedtuple return values at 922 and 923 

Please let me know if there are any issues and I will try my best to fix them ASAP.

Thank you!~ 