doc: update subprocess.killed

This commit changes the wording of subprocess.killed to reflect
that a child process was successfully signaled, and not
necessarily terminated.

Fixes: #16747
PR-URL: #16748
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/child_process.md

@@ -1048,10 +1048,11 @@ added: v0.5.10
 -->
 
 * {boolean} Set to `true` after `subprocess.kill()` is used to successfully
-  terminate the child process.
+  send a signal to the child process.
 
-The `subprocess.killed` property indicates whether the child process was
-successfully terminated using `subprocess.kill()`.
+The `subprocess.killed` property indicates whether the child process
+successfully received a signal from `subprocess.kill()`. The `killed` property
+does not indicate that the child process has been terminated.
 
 <a name="child_process_child_pid"></a>
 ### subprocess.pid