DOC: fix docstrings validation for pandas.core.groupby.DataFrameGroupBy.boxplot
Part of Issue #60365

Fixed docstring validation for pandas.core.groupby.DataFrameGroupBy.boxplot

The following errors were addressed:

- PR07    Parameter "grouped" has no description
- PR07    Parameter "fontsize" has no description
- PR07    Parameter "rot" has no description
- PR07    Parameter "grid" has no description
- PR07    Parameter "ax" has no description
- PR07    Parameter "figsize" has no description
- RT03    Return value has no description
- RT03    Return value has no description
- SA01    See Also section not found

Thanks @jmalp 