DOC: Fix docstrings for SeriesGroupBy monotonic and nth
part of #60365 

```
 -i "pandas.core.groupby.SeriesGroupBy.is_monotonic_decreasing SA01" \ 
 -i "pandas.core.groupby.SeriesGroupBy.is_monotonic_increasing SA01" \ 
 -i "pandas.core.groupby.SeriesGroupBy.nth PR02" \ 
 -i "pandas.core.groupby.DataFrameGroupBy.nth PR02" \
```
Thanks @ensalada-de-pechuga 