child_process: document kill() return value

This commit documents the return value from subprocess.kill().

PR-URL: #30669
Refs: #30668
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: cjihrig

doc/api/child_process.md

@@ -1061,10 +1061,12 @@ added: v0.1.90
 -->
 
 * `signal` {number|string}
+* Returns: {boolean}
 
 The `subprocess.kill()` method sends a signal to the child process. If no
 argument is given, the process will be sent the `'SIGTERM'` signal. See
-signal(7) for a list of available signals.
+signal(7) for a list of available signals. This function returns `true` if
+kill(2) succeeds, and `false` otherwise.
 
 ```js
 const { spawn } = require('child_process');