AK1003 - Add documentation (#7072)

* AK1003 - Add documentation

* Fix layout problem

docs/articles/debugging/rules/AK1003.md

@@ -0,0 +1,56 @@
+---
+uid: AK1003
+title: Akka.Analyzers Rule AK1003 - "ReceiveAsync<T>() or ReceiveAnyAsync<T>() message handler without async lambda body"
+---
+
+# AK1003 - Warning
+
+You should not have a synchronous code inside lambda function when using [`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
+
+Using `ReceiveAsync<T>()` or `ReceiveAnyAsync()` with synchronous code inside their lambda function is less performant compared to `Receive<T>()` or `ReceiveAny()` because they generates an extra `ActorTask` message plus an additional suspend + resume of the actor's mailbox. That overhead is fine if you're await-ing something, but if the code is essentially synchronous then you should not do so.
+
+An example:
+
+```csharp
+using Akka.Actor;
+using System.Threading.Tasks;
+using System;
+
+public sealed class MyActor : ReceiveActor
+{
+    public MyActor()
+    {
+        // Notice that there are no `await` inside the lambda function,
+        // The lambda function is essentially synchronous
+        ReceiveAsync<string>(async str => {
+            Sender.Tell(str);
+        }):
+    }
+}
+```
+
+## Resolution
+
+Use `Receive<T>()` or `ReceiveAny()` instead.
+
+Here's an example on how to fix the above example:
+
+```csharp
+using Akka.Actor;
+using System.Threading.Tasks;
+using System;
+
+public sealed class MyActor : ReceiveActor
+{
+    public MyActor()
+    {
+        // Notice we have replaced `ReceiveAsync` with `Receive`
+        // and removed the async keyword
+        Receive<string>(str => {
+            Sender.Tell(str);
+        }):
+    }
+}
+```