Document the "default NaN values" in Parser

[cancan101]

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.

[jreback]

this is in the main docs, I guess you could put in the doc string

[gfyoung]

@jreback : IINM, the docs now enumerate the default values. IMO, this is no longer an issue.

[jreback]

yes this is ok

[jorisvandenbossche]

Closed by #14030