doc: refactor signal info in child_process.md

* Since exec calls execFile and execFile internally calls spawn with
  options.signal, the signal parameter has been documented under exec
  as well.
* Refactor the description of signal under all the functions.
* Add examples of usage of signal under all the functions and add
  missing requires in the other examples.

PR-URL: https://github.com/nodejs/node/pull/37528


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>