domain: add message for dispose deprecation

PR-URL: https://github.com/nodejs/node/pull/7053
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/domain.js

@@ -314,4 +314,5 @@ Domain.prototype.dispose = util.deprecate(function() {
   // mark this domain as 'no longer relevant'
   // so that it can't be entered or activated.
   this._disposed = true;
-});
+}, 'Domain.dispose is deprecated. Recover from failed I/O actions explicitly ' +
+    'via error event handlers set on the domain instead.');