cleanup the custom block chapter

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed Tickets | -

This was showing incomplete code. Its still less than awesome, as the base sonata bundle provides an RSS block, and the Cmf BlockBundle also provides one which has a more complicated service, but still is not profiting from the fact that you could store data like the rss source in the block, making it rather pointless.

This PR at least makes the block in the doc more interesting. We might eventually want to update the CmfBlockBundle a bit too.