Add some info to MessagePack page (#19980)

Author: BrennanConroy

aspnetcore/signalr/messagepackhubprotocol.md

@@ -5,7 +5,7 @@ description: Add MessagePack Hub Protocol to ASP.NET Core SignalR.
 monikerRange: '>= aspnetcore-2.1'
 ms.author: bradyg
 ms.custom: mvc
-ms.date: 04/13/2020
+ms.date: 09/24/2020
 no-loc: ["ASP.NET Core Identity", cookie, Cookie, Blazor, "Blazor Server", "Blazor WebAssembly", "Identity", "Let's Encrypt", Razor, SignalR]
 uid: signalr/messagepackhubprotocol
 ---
@@ -57,6 +57,9 @@ services.AddSignalR()
 To enable MessagePack in the .NET Client, install the `Microsoft.AspNetCore.SignalR.Protocols.MessagePack` package and call `AddMessagePackProtocol` on `HubConnectionBuilder`.
 
 ```csharp
+using Microsoft.AspNetCore.SignalR.Client;
+using Microsoft.Extensions.DependencyInjection;
+
 var hubConnection = new HubConnectionBuilder()
                         .WithUrl("/chathub")
                         .AddMessagePackProtocol()
@@ -211,9 +214,11 @@ services.AddSignalR()
 ```
 
 > [!WARNING]
-> We strongly recommend reviewing [CVE-2020-5234](https://github.com/neuecc/MessagePack-CSharp/security/advisories/GHSA-7q36-4xx7-xcxf) and applying the recommended patches. For example, setting the `MessagePackSecurity.Active` static property to `MessagePackSecurity.UntrustedData`. Setting the `MessagePackSecurity.Active` requires manually installing a [1.9.x version of MessagePack](https://www.nuget.org/packages/MessagePack/1.9.3). Installing `MessagePack` 1.9.x upgrades the version SignalR uses. When `MessagePackSecurity.Active` is not set to `MessagePackSecurity.UntrustedData`, a malicious client could cause a denial of service. Set `MessagePackSecurity.Active` in `Program.Main`, as shown in the following code:
+> We strongly recommend reviewing [CVE-2020-5234](https://github.com/neuecc/MessagePack-CSharp/security/advisories/GHSA-7q36-4xx7-xcxf) and applying the recommended patches. For example, setting the `MessagePackSecurity.Active` static property to `MessagePackSecurity.UntrustedData`. Setting the `MessagePackSecurity.Active` requires manually installing a [1.9.x version of MessagePack](https://www.nuget.org/packages/MessagePack/1.9.3). Installing `MessagePack` 1.9.x upgrades the version SignalR uses. `MessagePack` version 2.x introduced breaking changes and is incompatible with SignalR versions 3.1 and earlier. When `MessagePackSecurity.Active` isn't set to `MessagePackSecurity.UntrustedData`, a malicious client could cause a denial of service. Set `MessagePackSecurity.Active` in `Program.Main`, as shown in the following code:
 
 ```csharp
+using MessagePack;
+
 public static void Main(string[] args)
 {
   MessagePackSecurity.Active = MessagePackSecurity.UntrustedData;
@@ -232,6 +237,9 @@ public static void Main(string[] args)
 To enable MessagePack in the .NET Client, install the `Microsoft.AspNetCore.SignalR.Protocols.MessagePack` package and call `AddMessagePackProtocol` on `HubConnectionBuilder`.
 
 ```csharp
+using Microsoft.AspNetCore.SignalR.Client;
+using Microsoft.Extensions.DependencyInjection;
+
 var hubConnection = new HubConnectionBuilder()
                         .WithUrl("/chathub")
                         .AddMessagePackProtocol()
@@ -389,6 +397,8 @@ services.AddSignalR()
 > We strongly recommend reviewing [CVE-2020-5234](https://github.com/neuecc/MessagePack-CSharp/security/advisories/GHSA-7q36-4xx7-xcxf) and applying the recommended patches. For example, setting the `MessagePackSecurity.Active` static property to `MessagePackSecurity.UntrustedData`. Setting the `MessagePackSecurity.Active` requires manually installing a [1.9.x version of MessagePack](https://www.nuget.org/packages/MessagePack/1.9.3). Installing `MessagePack` 1.9.x upgrades the version SignalR uses. When `MessagePackSecurity.Active` is not set to `MessagePackSecurity.UntrustedData`, a malicious client could cause a denial of service. Set `MessagePackSecurity.Active` in `Program.Main`, as shown in the following code:
 
 ```csharp
+using MessagePack;
+
 public static void Main(string[] args)
 {
   MessagePackSecurity.Active = MessagePackSecurity.UntrustedData;
@@ -407,6 +417,9 @@ public static void Main(string[] args)
 To enable MessagePack in the .NET Client, install the `Microsoft.AspNetCore.SignalR.Protocols.MessagePack` package and call `AddMessagePackProtocol` on `HubConnectionBuilder`.
 
 ```csharp
+using Microsoft.AspNetCore.SignalR.Client;
+using Microsoft.Extensions.DependencyInjection;
+
 var hubConnection = new HubConnectionBuilder()
                         .WithUrl("/chathub")
                         .AddMessagePackProtocol()