+clu #13584 Accept joining to be WeaklyUp during network split

* experimental feature, disabled by default
* Adding documentation to mention weakly up members.
  plus adding new diagram.

akka-cluster/src/main/scala/akka/cluster/ClusterEvent.scala

@@ -121,6 +121,16 @@ object ClusterEvent {
     def member: Member
   }
 
+  /**
+   * Member status changed to WeaklyUp.
+   * A joining member can be moved to `WeaklyUp` if convergence
+   * cannot be reached, i.e. there are unreachable nodes.
+   * It will be moved to `Up` when convergence is reached.
+   */
+  final case class MemberWeaklyUp(member: Member) extends MemberEvent {
+    if (member.status != WeaklyUp) throw new IllegalArgumentException("Expected WeaklyUp status, got: " + member)
+  }
+
   /**
    * Member status changed to Up.
    */
@@ -268,8 +278,9 @@ object ClusterEvent {
         case (_, newMember :: oldMember :: Nil) if newMember.status != oldMember.status ⇒ newMember
       }
       val memberEvents = (newMembers ++ changedMembers) collect {
-        case m if m.status == Up      ⇒ MemberUp(m)
-        case m if m.status == Exiting ⇒ MemberExited(m)
+        case m if m.status == WeaklyUp ⇒ MemberWeaklyUp(m)
+        case m if m.status == Up       ⇒ MemberUp(m)
+        case m if m.status == Exiting  ⇒ MemberExited(m)
         // no events for other transitions
       }