[core] Document the ActorRef<O> argument passed to O's constructor

src/mbgl/actor/actor.hpp

@@ -22,12 +22,6 @@ namespace mbgl {
  to R is *not* guaranteed (and can't be: S1 and S2 may be acting asynchronously with respect
  to each other).
 
- Construction and destruction of an actor is currently synchronous: the corresponding `O`
- object is constructed synchronously by the `Actor` constructor, and destructed synchronously
- by the `~Actor` destructor, after ensuring that the `O` is not currently receiving an
- asynchronous message. (Construction and destruction may change to be asynchronous in the
- future.)
-
  An `Actor<O>` can be converted to an `ActorRef<O>`, a non-owning value object representing
  a (weak) reference to the actor. Messages can be sent via the `Ref` as well.
 
@@ -38,6 +32,13 @@ namespace mbgl {
  the lifetime of the owning Actor, and sending a message to a `Ref` whose `Actor` has died is
  a no-op. (In the future, a dead-letters queue or log may be implemented.)
 
+ Construction and destruction of an actor is currently synchronous: the corresponding `O`
+ object is constructed synchronously by the `Actor` constructor, and destructed synchronously
+ by the `~Actor` destructor, after ensuring that the `O` is not currently receiving an
+ asynchronous message. (Construction and destruction may change to be asynchronous in the
+ future.) The constructor of `O` is passed an `ActorRef<O>` referring to itself (which it
+ can use to self-send messages), followed by the forwarded arguments passed to `Actor<O>`.
+
  Please don't send messages that contain shared pointers or references. That subverts the
  purpose of the actor model: prohibiting direct concurrent access to shared state.
 */