CLN: Update docstring decorator from Appender to doc
- [ ] xref #31942
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Attached are the screenshots of build docs

<img width="1256" alt="Screen Shot 2020-03-19 at 15 50 54" src="https://user-images.githubusercontent.com/34237447/77092398-b442a700-6a01-11ea-98ee-91e60acfb76a.png">

<img width="1254" alt="Screen Shot 2020-03-19 at 16 38 09" src="https://user-images.githubusercontent.com/34237447/77092667-e6540900-6a01-11ea-95a4-3f0cdcb1a804.png">

<img width="1248" alt="Screen Shot 2020-03-19 at 16 38 32" src="https://user-images.githubusercontent.com/34237447/77092688-eb18bd00-6a01-11ea-9c71-4f4d2f975309.png">

<img width="1246" alt="Screen Shot 2020-03-19 at 16 39 14" src="https://user-images.githubusercontent.com/34237447/77092716-f1a73480-6a01-11ea-928a-30d59a673755.png">

<img width="1252" alt="Screen Shot 2020-03-19 at 16 39 44" src="https://user-images.githubusercontent.com/34237447/77092741-f835ac00-6a01-11ea-8810-b092ad7b18c0.png">
 Thanks @RaisaDZ, nice clean up.