doc: add entry for subprocess.killed property

Backport-PR-URL: #14635
PR-URL: #14578
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

Author: Trott

doc/api/child_process.md

@@ -854,6 +854,17 @@ setTimeout(() => {
 }, 2000);
 ```
 
+### subprocess.killed
+<!-- YAML
+added: v0.5.10
+-->
+
+* {boolean} Set to `true` after `subprocess.kill()` is used to successfully
+  terminate the child process.
+
+The `subprocess.killed` property indicates whether the child process was
+successfully terminated using `subprocess.kill()`.
+
 <a name="child_process_child_pid"></a>
 ### subprocess.pid
 <!-- YAML