docs: Improved DI documentation (#2407)

docs/guides/dependency_injection/samples/enumeration.cs

@@ -0,0 +1,18 @@
+public class ServiceActivator
+{
+    // This contains *all* registered services of serviceType IService
+    private readonly IEnumerable<IService> _services;
+
+    public ServiceActivator(IEnumerable<IService> services)
+    {
+        _services = services;
+    }
+
+    public async Task ActivateAsync()
+    {
+        foreach(var service in _services)
+        {
+            await service.StartAsync();
+        }
+    }
+}