child_process: fix spawn and fork abort behavior

Fix AbortSignal in Spawn which doesn't actually abort
the process, and fork can emit an AbortError even if
the process was already exited. Add documentation
For killSignal.

Fixes: https://github.com/nodejs/node/issues/37273

PR-URL: https://github.com/nodejs/node/pull/37325


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>