docs: Improved DI documentation (#2407)

docs/guides/dependency_injection/samples/runasync.cs

@@ -0,0 +1,17 @@
+async Task RunAsync(string[] args)
+{
+    // Request the instance from the client.
+    // Because we're requesting it here first, its targetted constructor will be called and we will receive an active instance.
+    var client = _services.GetRequiredService<DiscordSocketClient>();
+
+    client.Log += async (msg) =>
+    {
+        await Task.CompletedTask;
+        Console.WriteLine(msg);
+    }
+
+    await client.LoginAsync(TokenType.Bot, "");
+    await client.StartAsync();
+
+    await Task.Delay(Timeout.Infinite);
+}