doc: note caveats in process message serialization

The message sent using process.send() goes through JSON
serialization and parsing, which could lead to surprising behaviors.
This commit elaborate a bit more on this and add a link to
the notes about these caveats in the ECMAScript specification.

PR-URL: https://github.com/nodejs/node/pull/12963
Refs: https://github.com/nodejs/node/issues/12497


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>