Docstring on Styler.apply{,map} is confusing
[Right now it says](https://github.com/pydata/pandas/blob/564166529071d33844712ab6038f6a237ddcb8f1/pandas/core/style.py#L357):

``` python
    def apply(self, func, axis=0, subset=None, **kwargs):
        """
        Apply a function column-wise, row-wise, or table-wase,
        updating the HTML representation with the result.
        .. versionadded:: 0.17.1
        Parameters
        ----------
        func: function
        axis: int, str or None
            apply to each column (``axis=0`` or ``'index'``)
            or to each row (``axis=1`` or ``'columns'``) or
            to the entire DataFrame at once with ``axis=None``.
        subset: IndexSlice
            a valid indexer to limit ``data`` to *before* applying the
            function. Consider using a pandas.IndexSlice
        kwargs: dict
            pass along to ``func``
        Returns
        -------
        self
        Notes
        -----
        This is similar to ``DataFrame.apply``, except that ``axis=None``
        applies the function to the entire DataFrame at once,
        rather than column-wise or row-wise.
        """
```

I spent ages trying to figure out how to feed any kind of function into this, only to eventually find the [Styler's docstring](https://github.com/pydata/pandas/blob/564166529071d33844712ab6038f6a237ddcb8f1/pandas/core/style.py#L68-L77):

``` python
    """
    Notes
    -----
    Most styling will be done by passing style functions into
    ``Styler.apply`` or ``Styler.applymap``. Style functions should
    return values with strings containing CSS ``'attr: value'`` that will
    be applied to the indicated cells.
    If using in the Jupyter notebook, Styler has defined a ``_repr_html_``
    to automatically render itself. Otherwise call Styler.render to get
    the genterated HTML.
    """
```

Really, the docstring of apply should state that the passed function should return CSS to be applied to the element. Otherwise, it just sounds like something which modifies the data before it is presented.

My use case is that I have a column full of [`uncertainties.ufloat`](https://pythonhosted.org/uncertainties/user_guide.html), and I want to convert them to a particular convenient string representation before they are formatted in a HTML table. I still can't figure out how to cleanly do this...
