Add example and fix for CS0409

docs/core/docker/snippets/App/Program.cs

@@ -1,9 +1,16 @@
 var counter = 0;
-var max = args.Length is not 0 ? Convert.ToInt32(args[0]) : -1;
 
-while (max is -1 || counter < max)
+// If a number is passed as a command-line argument,
+// it will be used as the maximum counter value.
+var max = args.Length > 0 && int.TryParse(args[0], out var parsedMax)
+    ? parsedMax
+    : -1;
+
+// Run indefinitely if no max value is provided
+while (max == -1 || counter < max)
 {
     Console.WriteLine($"Counter: {++counter}");
 
-    await Task.Delay(TimeSpan.FromMilliseconds(1_000));
+    // Wait for one second between iterations
+    await Task.Delay(TimeSpan.FromSeconds(1));
 }

docs/csharp/misc/cs0409.md

@@ -1,26 +1,48 @@
 ---
 description: "Compiler Error CS0409"
 title: "Compiler Error CS0409"
-ms.date: 07/20/2015
-f1_keywords: 
+ms.date: 09/15/2021
+f1_keywords:
   - "CS0409"
-helpviewer_keywords: 
+helpviewer_keywords:
   - "CS0409"
-ms.assetid: 23d86c13-7978-41b7-a087-ffcea52476fa
 ---
 # Compiler Error CS0409
 
-A constraint clause has already been specified for type parameter 'type parameter'. All of the constraints for a type parameter must be specified in a single where clause.  
-
- Multiple constraint clauses (where clauses) were found for a single type parameter. Remove the extraneous where clause, or correct the where clauses so that a unique type parameter in each clause.  
-
-```csharp  
-// CS0409.cs  
-interface I  
-{  
-}  
-
-class C<T1, T2> where T1 : I where T1 : I  // CS0409 – T1 used twice  
-{  
-}  
+A constraint clause has already been specified for type parameter 'type parameter'. All of the constraints for a type parameter must be specified in a single where clause.
+
+Multiple constraint clauses (`where` clauses) were specified for the same type parameter. To fix this error, remove the duplicate `where` clause, or change the constraint clauses so each one applies to a unique type parameter.
+
+## Example
+
+The following example generates CS0409:
+
+```csharp
+class Example<T>
+    where T : class
+    where T : new()
+{
+}
+```
+
+To fix this error, combine the constraints into a single clause:
+
+```csharp
+class Example<T>
+    where T : class, new()
+{
+}
+```
+
+The following sample also generates CS0409:
+
+```csharp
+// CS0409.cs
+interface I
+{
+}
+
+class C<T1, T2> where T1 : I where T1 : I  // CS0409 - T1 used twice
+{
+}
 ```