DOC: Validate reST directives in docstrings

#### Code Sample, a copy-pastable example if possible

```python
python scripts/validate_docstrings.py pandas.Series.item
```
#### Problem description
As pointed out by PR #27621, `scripts/validate_docstrings.py` does not detect dosctrings containing a reST directive without two colons.

#### Expected Output
The script should report an error in this case.
