doc: document missing properties in child_process

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

PR-URL: https://github.com/nodejs/node/pull/31342
Fixes: https://github.com/nodejs/node/issues/27206


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>