doc: update child_process.md

Add an explanation of the risk of exceeding platform pipe
capacity with uncaptured output in child_process.spawn
with stdio of pipe

PR-URL: https://github.com/nodejs/node/pull/19075
Fixes: https://github.com/nodejs/node/issues/4236


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>