doc: document missing properties in child_process

Add documentation for subprocess.exitCode, subprocess.signalCode,
subprocess.spawnargs, and subprocess.spawnfile.

PR-URL: #31342
Fixes: #27206
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/child_process.md

@@ -1055,6 +1055,13 @@ When the child process is a Node.js instance (e.g. spawned using
 [`child_process.fork()`][]), the `process.disconnect()` method can be invoked
 within the child process to close the IPC channel as well.
 
+### `subprocess.exitCode`
+
+* {integer}
+
+The `subprocess.exitcode` property indicates the exit code of the child process.
+If the child process is still running, the field will be null.
+
 ### `subprocess.kill([signal])`
 <!-- YAML
 added: v0.1.90
@@ -1348,6 +1355,34 @@ It is also recommended that any `'message'` handlers in the child process
 verify that `socket` exists, as the connection may have been closed during the
 time it takes to send the connection to the child.
 
+### `subprocess.signalCode`
+
+* {integer}
+
+The `subprocess.signalCode` property indicates the signal number received by
+the child process if any, else null.
+
+### `subprocess.spawnargs`
+
+* {Array}
+
+The `subprocess.spawnargs` property represents the full list of command line
+arguments the child process was launched with.
+
+### `subprocess.spawnfile`
+
+* {string}
+
+The `subprocess.spawnfile` property indicates the executable file name of
+the child process that is launched.
+
+For [`child_process.fork()`][], its value will be equal to
+[`process.execPath`][].
+For [`child_process.spawn()`][], its value will be the name of
+the executable file.
+For [`child_process.exec()`][],  its value will be the name of the shell
+in which the child process is launched.
+
 ### `subprocess.stderr`
 <!-- YAML
 added: v0.1.90