DOC Mention that loadings are found in the components_ attribute

Changed the docstring for the components_ attribute to mention that the loadings are found here.  
I find this description rather misleading, since it suggests that it makes sense to examine single entries of a principal axis, although the idea is to represent how features **co**vary "best". I would prefer something like

```
components_
    Principal axes, representing the directions (in feature space) of maximum variance in the data.
    These are also known as loading vectors.
```