DOC: correction of example in unstack docstring
There is an error in the example of the `unstack` docstring (http://pandas.pydata.org/pandas-docs/dev/generated/pandas.DataFrame.unstack.html ):

```
>>> df = s.unstack(level=0)
>>> df
   one  two
a  1.   2.
b  3.   4
```

should be 

```
   one  two
a  1.   3.
b  2.   4
```

So this PR fixes that, but in the same time I did some other adjustments. Mainly:
- added a "see also"
- added in the example the code to create the series, so one can run the example him/herself

@cpcloud OK, whitespace removed!
 @cpcloud okay to merge?
 yep
 thanks @jorisvandenbossche !
