DOC: read_fwf() is documented as supporting `delimiter`, but doesn't
The docstring and online manual for `read_fwf()` say:

> delimiter : str, default None
> Alternative argument name for sep.

But in fact, `sep` does nothing in this function, while `delimiter` has some effect, though perhaps not intentional:

    import io
    import pandas as pd

    pd.read_fwf(io.StringIO('foo,bar\n1,2'),delimiter=',')
    pd.read_fwf(io.StringIO('foo,bar\n1,2'),sep=',')

Both are nonsensical and should really produce an error, but they actually produce two different results:
```
   foo  bar
0  1,2  NaN
```
```
  foo,bar
0     1,2
```

The documentation should be cleaned up to not suggest the use of `delimiter` with `read_fwf()` unless it is actually useful and intended (in which case `sep` should be made to do the same, and documented).

Discovered by https://stackoverflow.com/questions/45257881/python-transform-string-into-dataframe-using-tab-as-separator/

Pandas 0.20: http://pandas.pydata.org/pandas-docs/version/0.20/generated/pandas.read_fwf.html