+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/routing/ClusterRouterConfig.scala

@@ -382,7 +382,7 @@ private[akka] trait ClusterRouterActor { this: RouterActor ⇒
   }
 
   def isAvailable(m: Member): Boolean =
-    m.status == MemberStatus.Up &&
+    (m.status == MemberStatus.Up || m.status == MemberStatus.WeaklyUp) &&
       satisfiesRole(m.roles) &&
       (settings.allowLocalRoutees || m.address != cluster.selfAddress)