DOC: Improvements in panel apply docstring
Not sure what the bar is for a PR here - this is a mild but meaningful improvement - in particular re the option to supply numbers & a tuple - by no means perfect though

@jreback green
 @jorisvandenbossche 
Thanks - I edited. I think we also can't just do `panel.apply`, but instead need the full expression `pd.Panel(pd.np.random.rand(4,3,2)).apply(pd.np.sqrt)`. Tell me if I'm wrong though
 @maximilianr In theory, a full expression is not needed as this code is at the moment never run in order to build the docs (as is the case with the tutorials), but it is always nice to have full examples so people can copy paste it.

For layout, I think something like this is best (once define the variable, and then use it instead repeating the creation):

```
Returns a Panel with the square root of each element

>>> p = pd.Panel(np.random.rand(4,3,2))
>>> p.apply(np.sqrt)

Equivalent to p.sum(1), returning a DataFrame

>>> p.apply(lambda x: x.sum(), axis=1)

Equivalent to previous:

>>> p.apply(lambda x: x.sum(), axis='minor')

....
```
 @jorisvandenbossche updated
 @jorisvandenbossche I accidentally PEP8-ed the whole file. Shall I undo, or is that actually helpful?
 @maximilianr yeh, normally want to do PEP stuff in separate PR's as confusing what is actually changed, but ok.
