Docstring validation script issues
#### Problem description

I am currently working on my PR for documentation sprint (for `pd.DataFrame.to_csv`)) and have a few issues with the docsting validation script, please see below. Master merged ~5 minutes ago, so everything should be up to date.

#### "Code"

My "Parameters" section starts with:

```Python
        Parameters
        ----------
        path_or_buf : str or file handle, default None
            File path or object, if None is provided the result is returned as
            a string.
        sep : str (length 1), default ','
            Field delimiter for the output file.
        na_rep : str, default ''
            Missing data representation.
```

#### Validation script outputs:

Run right one after another, without any change to the doc.

```
Errors found:
        Docstring text (summary) should start in the line immediately after the opening quotes (not in the same line, or leaving a blank line in between)
        Errors in parameters section
                Parameters {'decimal', 'sep', 'compression', 'date_format', 'doublequote', 'header', 'na_rep', 'quoting', 'line_terminator', 'quotechar', 'encoding', 'tupleize_cols', 'mode', 'chunksize', 'escapechar', 'index_label', 'float_format', 'columns', 'index'} not documented
                Unknown parameters {"'``"}
                Parameter "path_or_buf" description should start with capital letter
                Parameter "path_or_buf" description should finish with "."
                Parameter "'``" has no type
                Parameter "'``" description should start with capital letter
                Parameter "'``" description should finish with "."
        No returns section found
```

```
Errors found:
        Docstring text (summary) should start in the line immediately after the opening quotes (not in the same line, or leaving a blank line in between)
        Errors in parameters section
                Parameters {'index_label', 'float_format', 'decimal', 'header', 'encoding', 'chunksize', 'na_rep', 'doublequote', 'quotechar', 'sep', 'compression', 'tupleize_cols', 'quoting', 'line_terminator', 'date_format', 'mode', 'columns', 'escapechar', 'index'} not documented
                Unknown parameters {"'``"}
                Parameter "path_or_buf" description should start with capital letter
                Parameter "path_or_buf" description should finish with "."
                Parameter "'``" has no type
                Parameter "'``" description should start with capital letter
                Parameter "'``" description should finish with "."
        No returns section found
```

#### Expected Output

Issues found:

1. Docstring cannot start in the new line: when this is met, I got the following error:
```
Traceback (most recent call last):
  File "scripts/validate_docstrings.py", line 499, in <module>
    sys.exit(main(args.function))
  File "scripts/validate_docstrings.py", line 485, in main
    return validate_one(function)
  File "scripts/validate_docstrings.py", line 411, in validate_one
    doc.summary.split(' ')[0][-1] == 's'):
IndexError: string index out of range
```
2. `sep` is clearly documented, as far as I can say following [section-3-parameters here](https://python-sprints.github.io/pandas/guide/pandas_docstring.html#section-3-parameters);
3. "Not documented" parameters list changes with each script run;
4. `Unknown parameters {"'``"}` is caused by `line_terminator : str, default ``'\n'```, kind of fixed with `line_terminator : str, default '\\n'`, but this also doesn't seem to be correct;
5. Some other description issues like those with `path_or_buf`, which seem to be correct (?);
6. Return section seems to be optional ("If the method returns a value, it will be documented in this section." [here](https://python-sprints.github.io/pandas/guide/pandas_docstring.html#section-4-returns-or-yields), but the script lists this as error "No returns section found".

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: 2ec022f0d18014ff26f239a4f344f1e884e3d778
python: 3.6.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 142 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.0.dev0+539.g2ec022f0d.dirty

</details>
