Rename multicast function to broadcast

By definition, a broadcast emits messages to _all_ its recipients, which is not
necessarily true for a multicast. This change should avoid potential confusion
as to what the function actually does.
Improve coroutine documentation

Besides some rewrite, the coroutine docs have also been moved to the processes
docs. Although coroutines are an API, they are also heavily supposed to be used
by users. Therefore, it's better located there.
Merge pull request #100 from ufo-kit/multicast-rename-and-docs

Multicast rename and docs