Fixed to_string with line_width and without index
When calling to_string on a DataFrame with line_width, this calls
_join_multiline internally which only sets the idx variable when
index is True, otherwise idx is never set resulting in the error:

  UnboundLocalError: local variable 'idx' referenced before assignment

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13998?src=pr) is 85.27% (diff: 100%)

> Merging [#13998](https://codecov.io/gh/pydata/pandas/pull/13998?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
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  Files           139        139          
  Lines         50211      50212     +1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits          42819      42820     +1   
  Misses         7392       7392          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [5f49638...e0b1840](https://codecov.io/gh/pydata/pandas/compare/5f496380beb81d3986ca48cf41b893d95c227895...e0b1840d733d1c16f2307b8e03de9f610b5ced7c?src=pr)
 @cr3 Thanks! 
Can you add a small whatsnew notice (in docs/source/whatsnew/v0.19.0.txt, in the bug fixes section)

Also, I noticed `line_width` is not mentioned in the `to_string` docstring. Would you like to add an explanation for that as well?
 @jorisvandenbossche, thanks for the suggestion, I rebased my branch with both your suggestions. To be pedantic, I'm not sure if the description of `line_width` in the docstring should start with an upper or lower case character; the first descriptions in the docstring used lower case and the last ones used upper case, so I decided to be consistent with the last ones.
 > To be pedantic, I'm not sure if the description of line_width in the docstring should start with an upper or lower case character

No problem, I like that :-) Well, I think it should start with upper case, but we indeed do not follow this rigourously in the code base.
 @cr3 Thanks!
