CopyAzVHDFromVMToLab.ps1 - warn that source VM will be stopped for as long as the copy takes

This script shuts down the source VM and blocks from restarting, probably for over an hour. Please add a comment to warn users about that.

WRT script:
samples/DevTestLabs/Scripts/CopyVirtualMachines/CopyAzVHDFromVMToLab.ps1

While running it, I found out that the source VM will be stopped by the script for the duration of the script execution.

The main contributor to that time is the copying of the VHD. That took over 1 hour for a 256GB disk. There was little network traffic on my laptop running the script, so I presume the copy was happening in the cloud.

It seems the script effectively locks the source VHD so the source VM cannot be restarted. This is probably controlled by the parameter SignatureExpire which defaults to 3600 (seconds, or 1 hour).