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

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

Fixes: #5453
PR-URL: #5723
Reviewed-By: Jeremy Whitlock <jwhitlock@apache.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/child_process.markdown

@@ -902,6 +902,8 @@ tracking when the socket is destroyed. To indicate this, the `.connections`
 property becomes `null`. It is recommended not to use `.maxConnections` when
 this occurs.
 
+*Note: this function uses [`JSON.stringify()`][] internally to serialize the `message`.*
+
 ### child.stderr
 
 * {Stream}
@@ -996,3 +998,4 @@ to the same value.
 [`options.stdio`]: #child_process_options_stdio
 [`stdio`]: #child_process_options_stdio
 [synchronous counterparts]: #child_process_synchronous_process_creation
+[`JSON.stringify()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify

doc/api/process.markdown

@@ -841,6 +841,8 @@ When Node.js is spawned with an IPC channel attached, it can send messages to it
 parent process using `process.send()`. Each will be received as a
 [`'message'`][] event on the parent's `ChildProcess` object.
 
+*Note: this function uses [`JSON.stringify()`][] internally to serialize the `message`.*
+
 If Node.js was not spawned with an IPC channel, `process.send()` will be undefined.
 
 ## process.setegid(id)
@@ -1096,3 +1098,4 @@ Will print something like:
 [Signal Events]: #process_signal_events
 [Stream compatibility]: stream.html#stream_compatibility_with_older_node_js_versions
 [the tty docs]: tty.html#tty_tty
+[`JSON.stringify()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify