Documentation of differences in supported Trigger/Binding configuration sources when migrating

### What version of .NET does your existing project use?

.NET 6 In-Process

### What version of .NET are you attempting to target?

.NET 8 Isolated

### Description

We are in the process of migrating from .NET 6 In-Process to .NET 8 Isolated functions (running in App Plan/ASE), where our functions use trigger configuration (such as TimerTrigger schedules and ServiceBus connection strings) that are supplied by an Azure App Configuration setting. The Azure App Configuration was wired up in the FunctionStartup to act as a custom configuration source. This has all worked fine under the in-process model and our app service plan.

After migration to isolated model, none of these trigger settings worked.

We eventually found the reason in the Isolated documentation, under: https://learn.microsoft.com/en-us/azure/azure-functions/dotnet-isolated-process-guide?tabs=windows#configuration


There are also various Github issues explaining the reasoning why (#1884 and #1926) , but these indicate it would be the same in-process, but this scenario definitely worked previously (perhaps limitation is on consumption plan only?)

Please can this difference in supported configuration sources be added to the in-process to isolated migration documentation, as something to watch out for (i.e. custom configuration sources are not supported). For large function apps, this could be a significant issue to migrate settings to other methods of configuration.

Unfortunately, we are unable to migrate to App Configuration references as they do not currently support network-restricted appconfig stores.

### Project configuration and dependencies

_No response_

### Link to a repository that reproduces the issue

_No response_