Header customizations in Version 8 and above (#7013)

* Update header docs

* Add upgrade guide

* Apply suggestions from code review

Co-authored-by: Mauro Servienti <[email protected]>

---------

Co-authored-by: Mauro Servienti <[email protected]>

Author: andreasohlund

nservicebus/recoverability/configure-error-handling.md

@@ -30,7 +30,10 @@ include: configurationWarning
 
 ## Error message header customizations
 
-Before a message is moved to the error queue it is possible to inspect and modify its headers including [error forwarding headers](/nservicebus/messaging/headers.md#error-forwarding-headers).
+Before a message is moved to the error queue, it is possible to inspect and modify the [error forwarding headers](/nservicebus/messaging/headers.md#error-forwarding-headers).
+
+> [!WARNING]
+> Before Version 8, modifying existing headers on the failed message was possible. Starting in Version 8, use a [recoverability pipeline behavior](/nservicebus/recoverability/pipeline.md) to get full access to all headers. See the [upgrade guide](/nservicebus/upgrades/7to8/#header-manipulation-for-failed-messages) for more information.
 
 The following snippet shows how to configure header customizations and perform header value modification.
 

nservicebus/upgrades/7to8/index.md

@@ -203,6 +203,10 @@ endpointConfiguration.EnableOutbox();
 
 `ITransportReceiveContext.AbortReceiveOperation` has been deprecated in favor of throwing an [`OperationCanceledException`](https://docs.microsoft.com/en-us/dotnet/api/system.operationcanceledexception). This will preserve the NServiceBus version 7 behavior of immediately retrying the message without invoking [recoverability](/nservicebus/recoverability).
 
+## Header manipulation for failed messages
+
+In NServiceBus Version 8 the [header customization API](/nservicebus/recoverability/configure-error-handling.md#error-message-header-customizations) only allows manipulation of [error forwarding headers](/nservicebus/messaging/headers.md#error-forwarding-headers). Use the new [recoverability pipeline](/nservicebus/recoverability/pipeline.md) to get access to all headers on the failed message.
+
 ## Renamed extension method types
 
 The following static extension method types have been renamed: