DOC: Fix docstring validation errors for pandas.core.groupby

[milamathias]

• closes DOC: Fix docstring validation errors for pandas.core.groupby #60365
• 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.

Ran docstring validation for all methods in the issue fixed by other contributors:

scripts/validate_docstrings.py pandas.core.groupby.DataFrameGroupBy.boxplot
scripts/validate_docstrings.py pandas.core.groupby.DataFrameGroupBy.get_group
scripts/validate_docstrings.py pandas.core.groupby.DataFrameGroupBy.indices
scripts/validate_docstrings.py pandas.core.groupby.DataFrameGroupBy.nth
scripts/validate_docstrings.py pandas.core.groupby.DataFrameGroupBy.nunique
scripts/validate_docstrings.py pandas.core.groupby.DataFrameGroupBy.sem
scripts/validate_docstrings.py pandas.core.groupby.SeriesGroupBy.get_group
scripts/validate_docstrings.py pandas.core.groupby.SeriesGroupBy.indices
scripts/validate_docstrings.py pandas.core.groupby.SeriesGroupBy.is_monotonic_decreasing
scripts/validate_docstrings.py pandas.core.groupby.SeriesGroupBy.is_monotonic_increasing
scripts/validate_docstrings.py pandas.core.groupby.SeriesGroupBy.nth
scripts/validate_docstrings.py pandas.core.groupby.SeriesGroupBy.sem

And fixed the PR02 errors in:

 -i "pandas.core.groupby.DataFrameGroupBy.plot PR02" \ 
 -i "pandas.core.groupby.SeriesGroupBy.plot PR02" \ 

All validation tests now pass with the exception of a couple of ES01 errors, which can be ignored as stated. Issue #60365 can be closed after this PR merges.