DOC: ddof in cov when nans present
- [x] closes #47042
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This PR adds a document about the "``nan``s must not be present in the dataframe" restriction on ddof in pandas.DataFrame.cov.

This is my first time OSS contributing.
`Code Checks / pre-commit (pull_request)` is failing, what should I fix?
Please tell me!! Hey @whooolemilk Thanks for picking this up!

In general, how to work with errors (there will be many) is to look into the test logs and find the relevant log line. In this case try to look into the `Doc Build and Upload / Doc Build and Upload (pull_request)` one and you'll see an error message `rst inline code next to normal text`,i..e, its the "`nan`s" part that is the problem.  To avoid the rst check issue you could just reword the sentence, for example `This argument is applicable only when no ``nan`` is in the dataframe.` Looks good. Thanks @whooolemilk.  @topper-123 
Thanks for letting me know