DOC: Detect whether a Returns section is needed in validate_docstrings.py
One of the validations performed in the docstrings in `validate_docstrings.py` is whether a `Returns` section is present. So far we just check if the `return` word exist in the source code, and if it does, we generate an error if the docsting does not have the documentation section.

But this causes false positives, see this case:
```python
def say_hello(lang):
    """
    This function does not have a Returns section.

    This is correct because nothing is returned.

    Parameters
    ----------
    lang : str
         Language to say hello in.
     """
     if lang not in ('en', 'es'):
         return

    print({'en': 'Hi!', 'es': 'Hola!'}[lang])
```

The `return` keyword is found in the code, so the presence of a `Returns` section is validated, but it should not.

Without making anything complicated, it'd be nice to check when validating:
- That the return words found are not a bare return or a `return None` or are not comments...

If any code `return` is found that does not return always `None` is found, then we should show the error. 