doc: note synchronous part of child_process.spawn

If an application has a large RSS (GB), jorangreef
observed that child_process.spawn will block the
event loop for ms-s.

This is surprising behavior and merits documentation.

Refs: https://github.com/nodejs/node/issues/14917

PR-URL: https://github.com/nodejs/node/pull/21234


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>