Add async delegates to the overall list of migration considerations, and details in the details doc. (#34937)

* add note about async delegates https://devblogs.microsoft.com/dotnet/migrating-delegate-begininvoke-calls-for-net-core/ dotnet/runtime#16312 * other doc * feedback

Author: danmoseley

docs/core/porting/index.md

@@ -78,6 +78,8 @@ There are a few technologies in .NET Framework that don't exist in .NET:
 
  Remoting is used for communicating across application domains, which are no longer supported. For simple communication across processes, consider inter-process communication (IPC) mechanisms as an alternative to remoting, such as the <xref:System.IO.Pipes> class or the <xref:System.IO.MemoryMappedFiles.MemoryMappedFile> class. For more complex scenarios, consider frameworks such as [StreamJsonRpc](https://github.com/microsoft/vs-streamjsonrpc) or [ASP.NET Core](/aspnet/core) (either using [gRPC](/aspnet/core/grpc) or [RESTful Web API services](/aspnet/core/web-api)).
 
+ Because remoting is not supported, calls to `BeginInvoke()` and `EndInvoke()` on delegate objects will throw `PlatformNotSupportedException`.
+
 - [Code access security (CAS)](net-framework-tech-unavailable.md#code-access-security-cas)
 
  CAS was a sandboxing technique supported by .NET Framework but deprecated in .NET Framework 4.0. It was replaced by Security Transparency and it isn't supported in .NET. Instead, use security boundaries provided by the operating system, such as virtualization, containers, or user accounts.

docs/core/porting/net-framework-tech-unavailable.md

@@ -27,6 +27,8 @@ Across machines, use a network-based solution as an alternative. Preferably, use
 
 For more messaging options, see [.NET Open Source Developer Projects: Messaging](https://github.com/Microsoft/dotnet/blob/master/dotnet-developer-projects.md#messaging).
 
+Because remoting is not supported, calls to `BeginInvoke()` and `EndInvoke()` on delegate objects will throw `PlatformNotSupportedException`. For more information, see [Migrating Delegate BeginInvoke Calls For .NET Core](https://devblogs.microsoft.com/dotnet/migrating-delegate-begininvoke-calls-for-net-core/).
+
 ## Code access security (CAS)
 
 Sandboxing, which relies on the runtime or the framework to constrain which resources a managed application or library uses or runs, [isn't supported on .NET Framework](/previous-versions/dotnet/framework/code-access-security/code-access-security) and therefore is also not supported on .NET 6+. CAS is no longer treated as a security boundary, because there are too many cases in .NET Framework and the runtime where an elevation of privileges occurs. Also, CAS makes the implementation more complicated and often has correctness-performance implications for applications that don't intend to use it.