DOC: Improve DataFrameGroupBy.std ddof doc

[KevsterAmp]

• closes DOC: std() delta degree of freedom vs. degree of freedom #59405 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations 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.

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

[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

[mroeschke]

Thanks @KevsterAmp

[rhshadrach]

@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!