DOC: Removed numeric_only parameter from pd.DataFrame.mad docs
- closes #29079 

Please review this @MarcoGorelli @mroeschke  I think it's fine, could you include a screenshot of the documentation?

Inside the `doc` folder, run
```
python make.py clean
python make.py --single pandas.DataFrame.mad
```

(see [building the documentation](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#building-the-documentation)) ![Screenshot from 2020-02-05 04-59-59](https://user-images.githubusercontent.com/39258575/73796968-bf849f00-47d4-11ea-844b-045458f298c1.png)

Hi, I'm getting the following error when I run "python make.py clean"
When I ran "python3 make.py clean", I got an error saying "ModuleNotFoundError: No module named 'pandas._libs.tslibs.conversion' " @r0cketr1kky you'll need to build the c extensions: https://pandas-docs.github.io/pandas-docs-travis/development/contributing.html#creating-a-python-environment Documentation screenshot @MarcoGorelli : 

![Screenshot from 2020-02-05 06-37-33](https://user-images.githubusercontent.com/39258575/73802064-6a4f8a00-47e2-11ea-9158-919b36cd1c03.png)

P.S. Thanks for the link @mroeschke . This helped me. Please review this! How long does it take to merge this PR? @r0cketr1kky I don't have write-access to the repo, and I presume that those who do are prioritising other work. Please be patient, I'm pretty sure this will get merged Sure, thanks a lot for assisting me! thanks @r0cketr1kky 

if you'd canvas the other operations that use ``_num_doc`` to see if any need changing would be great. Ahh I just saw this comment @jreback I'm having a busy semester :(
Nevertheless thank you @Bharat123rox for this commit :+1: 