doc: fix module.children description

Updates the module.children description in the docs to match current
functionality in that it will only contain the modules required for the
first time by this module.

Fixes: https://github.com/nodejs/node/issues/9371

PR-URL: https://github.com/nodejs/node/pull/21672


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>