DOC/CLN: Fix errors in Series docstrings
Fixes (or starts to fix) a few errors in `Series` docstrings.  On a related note, it seems `validate_docstrings.py` yields this error for a large number of docstrings:

```
The first line of the Returns section should contain only the type, unless multiple values are being returned
```

which seems to happen whenever the return value is named.  Should these be errors?