DOC: update docs for read_csv().na_values and keep_default_na
- [x] closes #13967 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14030?src=pr) is 85.25% (diff: 100%)

> Merging [#14030](https://codecov.io/gh/pydata/pandas/pull/14030?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14030   diff @@
==========================================
  Files           139        139          
  Lines         50380      50383     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42953      42955     +2   
- Misses         7427       7428     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [453bc26...ff1964b](https://codecov.io/gh/pydata/pandas/compare/453bc26e07ebbfea33dda9ff2f4efab683718506...ff1964b51fe9a2a45a9e4783fa979bebd4390772?src=pr)
 Do I need add `what's new` for this? And if so, in which `what's new` file?

Also I am not sure about this error:

```
2016-08-18 18:50:05 (7.84 MB/s) - 'miniconda.sh' saved [21310143/21310143]
ERROR: File or directory already exists: /Users/travis/miniconda
```
 whatsnew is not needed. And don't worry about the travis error, that is something unrelated.
 @jorisvandenbossche Hey I made some changes. Is there anyway I can preview it?
 You can just look at the docstring in the console to see if it looks ok (with ipython: `pd.read_excel?`)
 I squashed and removed the backticks.

I did test:

``` py
In [12]: _NA_VALUES = set([
   ....:     '-1.#IND', '1.#QNAN', '1.#IND', '-1.#QNAN', '#N/A N/A', '#N/A',
   ....:     'N/A', 'NA', '#NA', 'NULL', 'NaN', '-NaN', 'nan', '-nan', ''
   ....: ])

In [13]: s = """as NaN: '""" + "', '".join(sorted(_NA_VALUES)) + """'."""

In [14]: print s
as NaN: '', '#N/A', '#N/A N/A', '#NA', '-1.#IND', '-1.#QNAN', '-NaN', '-nan', '1.#IND', '1.#QNAN', 'N/A', 'NA', 'NULL', 'NaN', 'nan'.
```

I didn't check the doc with installation and `pd.read_excel?` because I start getting [weird error](http://pastebin.com/kBHCAHbc) out of nowhere. I am trying to fix it but I have no clue how this is happening yet. So should I post the problem in somewhere if I have no luck hacking it? Like google group or irc channel? Thanks!
 Okay I fixed it. Something's wrong with Cython. 
Ready to merge!
 thanks!

Can you do a followup where we make this wrap on the NaNs (they generate an extra long line)

```
na_values : str or list-like or dict, default None
    Additional strings to recognize as NA/NaN. If dict passed, specific
    per-column NA values. By default the following values are interpreted
    as NaN: '', '#N/A', '#N/A N/A', '#NA', '-1.#IND', '-1.#QNAN', '-NaN', '-nan', '1.#IND', '1.#QNAN', 'N/A', 'NA', 'NULL', 'NaN', 'nan'.
thousands : str, default None
    Thousands separator for parsing string columns to numeric.  Note that
    this parameter is only necessary for columns stored as TEXT in Excel,
    any numeric columns will automatically be parsed, regardless of display
    format.
```
 Sure I'll try!
