doc: update spawnSync() status value possibilities

The object returned by `child_process.spawnSync()` can have the `status`
property set to `null` if the process terminated due to a signal. We
even test for this in
test/parallel/test-child-process-spawnsync-kill-signal.js.

Update the documentation to reflect this.

PR-URL: nodejs/node#26680
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/child_process.md

@@ -868,8 +868,10 @@ changes:
   * `output` {Array} Array of results from stdio output.
   * `stdout` {Buffer|string} The contents of `output[1]`.
   * `stderr` {Buffer|string} The contents of `output[2]`.
-  * `status` {number} The exit code of the child process.
-  * `signal` {string} The signal used to kill the child process.
+  * `status` {number|null} The exit code of the subprocess, or `null` if the
+    subprocess terminated due to a signal.
+  * `signal` {string|null} The signal used to kill the subprocess, or `null` if
+    the subprocess did not terminate due to a signal.
   * `error` {Error} The error object if the child process failed or timed out.
 
 The `child_process.spawnSync()` method is generally identical to