DOC: Improve DataFrameGroupBy.std ddof doc
- [x] closes #59405 (Replace xxxx with the GitHub issue number)
- [x] [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
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Modified ddof parameter based on `Dataframe.std` ddof doc
https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.std.html#pandas.DataFrame.std

<img width="762" alt="image" src="https://github.com/user-attachments/assets/5a65bb3b-7aa2-4bbb-9f7d-58b22a42bf14">


@rhshadrach - Saw a couple of DataFrameGroupBy functions with the similar "Degrees of Freedom" to the current .std(). Should I change it as well to copy the doc on DataFrame (ie DataFrame.var)

https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.DataFrameGroupBy.sem.html
https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.DataFrameGroupBy.var.html Thanks @KevsterAmp  > @rhshadrach - Saw a couple of DataFrameGroupBy functions with the similar "Degrees of Freedom" to the current .std(). Should I change it as well to copy the doc on DataFrame (ie DataFrame.var)
> 
> https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.DataFrameGroupBy.sem.html https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.DataFrameGroupBy.var.html

Yep!