DOC: scripts/validate_docstrings.py should detect when the short summary span over multiple lines
#### Code Sample, a copy-pastable example if possible

Take any docstring and write a short summary (the first line of the docstring) on multiple lines. Then test it with `./scripts/validate_docstrings.py pandas.DataFrame.my_method`. 

#### Problem description

As discussed in #22516 and in the [documentation](https://pandas.pydata.org/pandas-docs/stable/contributing_docstring.html), the first line of the docstring should be a short summary and shouldn't take more than one line.

#### Expected Output

An error stating that the first line (the short summary) should not be more than one line long.
