DOCS: fix docstring validation errors for pandas.Series
Partially addresses #59592 

Fixes:
```
 -i "pandas.Series.pop SA01" \
 -i "pandas.Series.list.__getitem__ SA01" \
 -i "pandas.Series.list.flatten SA01" \
 -i "pandas.Series.list.len SA01" \
 -i "pandas.Series.reorder_levels RT03,SA01" \
 -i "pandas.Series.sparse.density SA01" \
 -i "pandas.Series.gt SA01" \
 -i "pandas.Series.lt SA01" \
 -i "pandas.Series.ne SA01" \
 -i "pandas.Series.prod RT03" \
 -i "pandas.Series.product RT03" \
```
Thanks @hlakams 