CLN: DataFrame move doc from __init__ to cls

This, in part addresses our discussion from #3337.
This PR moves the doc string  from __init__ to the more common class location.
(cf.. numpy, rest of pandas classes that have a doc string)
Besides STY / consistency  this would allow for more directly accessing the doc string in a debugging (pdb) session: 

``` Python
print mydf.__doc__
```