[MRG] DOC improve svm sample weight example

Fixes issue #2191.
The sample weights doc and example were not terribly informative imho.
I tried to improve it.
The example looked like this:
![plot_weighted_samples_1](https://f.cloud.github.com/assets/449558/861421/a03f839c-f5d0-11e2-95cb-f24bfaa40252.png)

Now it looks like this:
![class_weight_example](https://f.cloud.github.com/assets/449558/861516/1e523b2e-f5d3-11e2-9f43-ffb9045271c4.png)


I'll add another sentence to the docs in a minute.