DOC: pd.read_csv doc-string clarification #11555
closes #11555 
Updated IO Tools documentation for read_csv() and read_table() to be consistent with the doc-string,
also reordered keywords to group them more logically. Also updated merging.rst docs for concat.

Migrated from https://github.com/pydata/pandas/pull/11756

Here's what the IO docs would look like:

![screen shot 2016-02-07 at 9 30 19 pm](https://cloud.githubusercontent.com/assets/5619798/12879722/e10f86d2-cdec-11e5-833e-ac1d5f2d061f.png)
 looks like a couple of linting errors

```
linting -> pandas/io
pandas/io/parsers.py:51:80: E501 line too long (80 > 79 characters)
pandas/io/parsers.py:54:80: E501 line too long (80 > 79 characters)
pandas/io/parsers.py:62:80: E501 line too long (80 > 79 characters)
pandas/io/parsers.py:67:80: E501 line too long (80 > 79 characters)
```
 Fixed long line lint errors.
 ok, ping when green.

I think we can merge this.

@jorisvandenbossche ok ?
 @frankcleary actually, pls add a note in whatsnew, maybe in Enhancements (with a link to the docs)
 thanks @frankcleary 

when docs are built (prob a few hours), have a look: http://pandas-docs.github.io/pandas-docs-travis/

and if needed pls issue a follow up to correct
 @frankcleary looks good!

@jorisvandenbossche 
was wondering if we should put references (this for io.rst) in the 'doc-string' area to some of the examples
e.g. http://pandas-docs.github.io/pandas-docs-travis/io.html#na-values could be linked from the `na_values` option.

?
