doc: update Diagnostics WG info

Updates info on Diagnostics WG (https://github.com/nodejs/diagnostics).

PR-URL: https://github.com/nodejs/node/pull/9329


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>