doc: small improvements of core cluster pages (#27939)

* proofreading of core cluster pages
* some more info in failure detector

akka-cluster-typed/src/test/java/jdocs/akka/cluster/typed/BasicClusterExampleTest.java

@@ -7,6 +7,7 @@ package jdocs.akka.cluster.typed;
 // #join-seed-nodes
 import akka.actor.Address;
 import akka.actor.AddressFromURIString;
+import akka.cluster.Member;
 import akka.cluster.typed.JoinSeedNodes;
 
 // #join-seed-nodes
@@ -120,4 +121,29 @@ public class BasicClusterExampleTest { // extends JUnitSuite {
     Cluster.get(system).manager().tell(new JoinSeedNodes(seedNodes));
     // #join-seed-nodes
   }
+
+  static class Backend {
+    static Behavior<Void> create() {
+      return Behaviors.empty();
+    }
+  }
+
+  static class Frontend {
+    static Behavior<Void> create() {
+      return Behaviors.empty();
+    }
+  }
+
+  void illustrateRoles() {
+    ActorContext<Void> context = null;
+
+    // #hasRole
+    Member selfMember = Cluster.get(context.getSystem()).selfMember();
+    if (selfMember.hasRole("backend")) {
+      context.spawn(Backend.create(), "back");
+    } else if (selfMember.hasRole("front")) {
+      context.spawn(Frontend.create(), "front");
+    }
+    // #hasRole
+  }
 }

akka-cluster-typed/src/test/scala/docs/akka/cluster/typed/BasicClusterExampleSpec.scala

@@ -62,6 +62,27 @@ akka {
     Cluster(system).manager ! JoinSeedNodes(seedNodes)
     //#join-seed-nodes
   }
+
+  object Backend {
+    def apply(): Behavior[_] = Behaviors.empty
+  }
+
+  object Frontend {
+    def apply(): Behavior[_] = Behaviors.empty
+  }
+
+  def illustrateRoles(): Unit = {
+    val context: ActorContext[_] = ???
+
+    //#hasRole
+    val selfMember = Cluster(context.system).selfMember
+    if (selfMember.hasRole("backend")) {
+      context.spawn(Backend(), "back")
+    } else if (selfMember.hasRole("frontend")) {
+      context.spawn(Frontend(), "front")
+    }
+    //#hasRole
+  }
 }
 
 class BasicClusterConfigSpec extends WordSpec with ScalaFutures with Eventually with Matchers with LogCapturing {