Add documentation regarding the scope of TunnelId

[rmmason]

Could you please add some documentation regarding the scope and intended purpose of the Tunnel Id beyond giving an id to refer to on the command line.

For instance, we have just started exploring dev-tunnels. My colleague created a tunnel called "tunnel-123". I then utilised his instructions to create "tunnel-123" on my machine under a different user account and get the error:

"Tunnel service error: Conflict with existing entity. Retry tunnel operation." 
Request ID: 75749124-1a98-45ad-89b2-1bf6dce589b7

It is not clear from the documentation at what scope the Tunnel ID needs to be unique. Is this globally across all clusters? Maybe it is unique per cluster? Maybe it is based on our Entra Tennant?

Knowing the scope of the Tunnel ID would be useful so that we can create names that do not conflict.