doc: Add note about use of JSON.stringify()

process.send and child.send use JSON.stringify to serialize
the message.

Fixes: https://github.com/nodejs/node/issues/5453
PR-URL: https://github.com/nodejs/node/pull/5723


Reviewed-By: Jeremy Whitlock <jwhitlock@apache.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>