DOC: correction of example in unstack docstring

[jorisvandenbossche]

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]

less than minor point: can u kill this white space here? thx. then i'll merge it

[jorisvandenbossche]

@cpcloud OK, whitespace removed!

[jtratner]

@cpcloud okay to merge?

[cpcloud]

yep

[jtratner]

thanks @jorisvandenbossche !