Document router handling of PoisonPill messages. Fixes #2874

DRAFT - I still need to modify the Java docs. I'd also like to include a more concrete description about how the router supervision hierarchy works, but I only partly understood the description in the docs. It would be good to understand more about that—I might post an akka-user question about it and possibly update with any clarifications. A diagram might help too!

* Revamped description of Broadcast messages.
* A few other minor doc edits.
* Removed border, background colour from literals' CSS, so literals with whitespace don't look silly.