doc: warn about unvalidated input in child_process

child_process.exec*() and child_process.spawn*() (if options.shell is
true) allow trivial arbitrary command execution if code passes
unsanitised user input to it. Add warnings in the docs to make that
clear.

PR-URL: https://github.com/nodejs/node/pull/10466


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>