Series.fillna() docs still claim self is return if inplace=True
In master, `NDFrame.fillna`'s `inplace` parameter's docs claim:

> If True, fill in place. Note: this will modify any
> other views on this object, (e.g. a no-copy slice for a column in a
> DataFrame).  **Still returns the object.**

But this is not consistently true. If `inplace=True`, `self` is only returned on a couple of paths. In the typical case of `s = Series(); s.fillna(-1, inplace=True)`, `None` is returned.

In 0.12.0 (and judging by a warning in an earlier version, from 0.11), `Series.fillna` consistently (and very deliberately) returns `None` if `inplace=True`, despite its docs claiming something similar:

> If True, fill the Series in place. Note: this will modify any other
> views on this Series, for example a column in a DataFrame. Returns
> a reference to the filled object, **which is self if inplace=True**

So I believe the intended behaviour is to consistently return `None` if `inplace=True`, and the docs are simply out of date.
