Add documentation about DELETE /session

@ametaireau r?
From what I understood, DND is handled on the client side, not on the server side. When a client unregisters from the server it means the server is not able to reach to it. What happens in case the client is joined is up to the client.

Since "DND" is already used with a different meaning on the client side, I would rather explicit that the server is not able to join the client anymore rather than reusing a term and changing slightly its meaning.