doc: shell option for the execFile and execFileSync functions

Useful for executing in a shell because it accepts arguments as
an array instead of a string as exec does.
Depending on the circumstances,
that can prove to be useful if the arguments are already prepared.

PR-URL: https://github.com/nodejs/node/pull/18237
Fixes: https://github.com/nodejs/node/issues/18199


Reviewed-By: Julian Duque <julianduquej@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Adrian Estrada <edsadr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>