DOC: Fix some docstring validations in pd.Series
Partially addresses #59592 

Fixes:
```
-i "pandas.Series.dt.unit GL08" \
-i "pandas.Series.pad PR01,SA01" \
```

- I have documented `pandas.Series.dt.uni`
- The docstring for `pandas.Series.pad` is unecesary as it even fails on the docstring check:

![image](https://github.com/user-attachments/assets/20e7d0de-1f7c-452c-98b0-fda0db679e98)


Thanks @OscarGB 