DataFrame.apply documentation needs upgrade
#### Problem description

I believe the only place that `apply` is mentioned in detail in the documentation is in the [essential basic functionality](http://pandas.pydata.org/pandas-docs/stable/basics.html#row-or-column-wise-function-application). 

All the examples do not need `apply`, and can be done quicker with other methods. In fact, they actively teach how not to use pandas correctly. For instance, `idxmax` and `interploate` are both dataframe methods and from the documentation, they appear to be good use-cases for `apply`, which they are not.

#### Expected Output

I understand that it makes sense to use basic examples to motivate `apply`.  But there should be some useful examples where `apply` is necessary. 

Most of the examples should be replaced (or shown how to be duplicated with more idiomatic methods).


