Warn when az container exec with --os-type windows

**Is your feature request related to a problem? Please describe.**

If I try to run Powershell in a container like this ⬇️ it just exits immediately with no feedback.

```sh
$ az container create --os-type windows --resource-group dev --name win-sandbox --image mcr.microsoft.com/windows/servercore:1809
$ az container exec -g dev --name win-sandbox --exec-command "powershell.exe"
$ # back at prompt immediately with no feedback

```
This is hidden in [the docs](https://docs.microsoft.com/en-us/cli/azure/container?view=azure-cli-latest#az_container_exec) ⬇️, but it took a [StackOverflow question](https://stackoverflow.com/questions/70148536/run-interactive-powershell-in-windows-azure-container-instance/70149465#70149465) to lead me there.
> This command is currently not supported for Windows machines.

**Describe the solution you'd like**

Ideally it would Just Work, but assuming that's not coming soon, then a warning / explanation of what happened would be nice.