Set-AzApplicationGatewayBackendAddressPool - Documented Parameter Not Exists

I couldn't find a note about templates for documentation questions/issues in the contributing guide. Apologies if not formatted correctly.

In the documentation for Set-AzApplicationGatewayBackendAddressPool ( [link](https://docs.microsoft.com/en-us/powershell/module/az.network/set-azapplicationgatewaybackendaddresspool?view=azps-5.3.0) and [source](https://github.com/Azure/azure-powershell/blob/master/src/Network/Network/help/Set-AzApplicationGatewayBackendAddressPool.md) ) it says in the Description:

> The Set-AzApplicationGatewayBackendAddressPool cmdlet updates a back-end address pool for an Azure application gateway. Back-end addresses can be specified as IP addresses, fully-qualified domain names (FQDN) or IP configurations IDs.

Specifically the **or IP configurations IDs.** portion. This functionality would be really helpful when configuring the backend pools as it's the property that is returned in the `Get-AzApplicationGatewayBackendAddressPool` cmdlet, maybe the functionality is in the pipeline? Looking at what's in master, there is no parameter declared for the IP configuration IDs and the logic does not seem to allow for one.

I am happy to submit a PR to remove this from the documentation if this feature is not coming online soon. It's a little confusing as I wasted a bit of time trying to use the IP configuration Ids.

Open src/Network/Network/help/Set-AzApplicationGatewayBackendAddressPool.md and compare its documented parameters and description with the Set-AzApplicationGatewayBackendAddressPool implementation in the repository. Confirm whether IP configuration IDs are supported; done means the documentation accurately reflects the available functionality and no longer misleads users.

Written by the indexing model from the issue text.