some scaladoc quirks

Hi Gilles,

I came across some issues with scaladoc:

1) Parameter doc comment is screwed up when inherited. You can see this
in the doc comment for method
ReplyReactor#!: Future[A]

If you expand the doc, it says "@param msg the message to be sent",
which should be rendered as a list item, of course.

2) Sometimes non-visible types are shown. For example, the documentation
for the `Actor` trait says
trait Actor extends AbstractActor with ReplyReactor
with ReplyableActor

Since trait `ReplyableActor` is `private[actors]`, it should not
appear in scaladoc IMO (just like a value member that is
`private[actors]`).

Cheers,
Philipp