DOC: `ddof` in `cov` when `nan`s present

[attack68]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.cov.html

Documentation problem

The ddof argument is applicable only when nans are not present in the dataframe.
It can be quite common to calculate a covariance matrix from financial data where the last row might be nan since the difference is taken between values. One might expect that since the result is given the ddof is adhered to. But it is not. I think this should be explicitly stated in the argument documentation, rather than hidden in the notes.

Suggested fix for documentation

.

[topper-123]

Yeah, if would fine to have mentioned in the ddof doc line. Contributions welcome.

[ma-ma-hima]

take

[ma-ma-hima]

@attack68 @topper-123
May I add the following text?

The ddof argument is applicable only when nans are not present in the dataframe.

[topper-123]

I think that's a good line to add.