Add axis argument to DataFrame.corr
#### Location of the documentation

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.corr.html

#### Documentation problem

How should I get the correlation among the rows? I can obviously do `df.T.corr()` but this strikes me as a workaround rather than a nice way to do things. I wrote a `df.corr(axis=1)` assuming that would work. Curious what people feel is idiomatic, and would recommend some mention in the docs. 