Document the "default NaN values" in Parser
For example from docs (http://pandas.pydata.org/pandas-docs/stable/generated/pandas.io.parsers.read_csv.html):

```
keep_default_na : bool, default True
If na_values are specified and keep_default_na is False the default NaN values are overridden, otherwise they’re appended to
```

But it is not clear what those default values are.
