Add documentation for AK1002 (#7066)

* Add documentation for AK1002

* Fix wrong heading

* Fix wrong header

docs/articles/debugging/rules/AK1002.md

@@ -0,0 +1,52 @@
+---
+uid: AK1002
+title: Akka.Analyzers Rule AK1002 - "Must not await `Self.GracefulStop()` inside `ReceiveAsync` or `ReceiveAnyAsync`"
+---
+
+# AK1002 - Error
+
+You should never await [`Self`](xref:Akka.Actor.ActorBase#Akka_Actor_ActorBase_Self).[`GracefulStop()`](xref:Akka.Actor.GracefulStopSupport#Akka_Actor_GracefulStopSupport_GracefulStop_Akka_Actor_IActorRef_System_TimeSpan_) inside [`ReceiveActor`](xref:Akka.Actor.ReceiveActor) [`ReceiveAsync<T>()`](xref:Akka.Actor.ReceiveActor#Akka_Actor_ReceiveActor_ReceiveAsync__1_System_Func___0_System_Threading_Tasks_Task__System_Predicate___0__) or [`ReceiveAnyAsync()`](xref:Akka.Actor.ReceiveActor#Akka_Actor_ReceiveActor_ReceiveAnyAsync_System_Func_System_Object_System_Threading_Tasks_Task__)
+
+## Cause
+
+Awaiting `Self.GracefulStop()` inside `ReceiveAsync<T>()` or `ReceiveAnyAsync()` will cause a deadlock because the `ReceiveActor` will block and wait inside the message handler for itself to terminate while its `PoisonPill` signal is stuck inside its `MailBox`, waiting to be processed.
+
+An example:
+
+```csharp
+using Akka.Actor;
+using System.Threading.Tasks;
+using System;
+
+public sealed class MyActor : ReceiveActor
+{
+   public MyActor()
+   {
+      ReceiveAsync<string>(async str => {
+         await Context.Self.GracefulStop(); // THIS WILL DEADLOCK
+      }):
+   }
+}
+```
+
+## Resolution
+
+If you absolutely need to invoke `Self.GracefulStop()` inside `ReceiveAsync<T>()` or `ReceiveAnyAsync()`, make sure that you're using a detached `Task` instead of awaiting for it to complete.
+
+Here's an example below:
+
+```csharp
+using Akka.Actor;
+using System.Threading.Tasks;
+using System;
+
+public sealed class MyActor : ReceiveActor
+{
+   public MyActor()
+   {
+      ReceiveAsync<string>(async str => {
+         _ = Context.Self.GracefulStop();
+      }):
+   }
+}
+```