[MRG+1] Fix FeatureAgglomeration docs

The docs are not displayed properly here, http://scikit-learn.org/stable/modules/generated/sklearn.cluster.FeatureAgglomeration.html (I keep running into these)
> What was your intuition behind this?

Basically, the attribute does not exist on that class, only on its parent class, and Python has no way to mark an attribute as deleted; it only has a way to override them.

I've not read it, but something like http://www.cafepy.com/article/python_attributes_and_methods/python_attributes_and_methods.html might help explain. Basically, each object (including classes) may have a special `__dict__` mapping providing settings for a dynamic set of attributes, and all other members (including methods, `__slots__`, etc) are delegated to descriptors on the object's class (i.e. found in its `__dict__`).