doc: deprecate debug protocol

Due to changes in V8, the debug protocol will no longer work in Node.js
8.0.0. Note this in the documentation.

PR-URL: https://github.com/nodejs/node/pull/10320


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>