tty,doc: add type-check to isatty

Previously, various inputs other than non-negative integers would
produce incorrect results.

Added type-checking on input, returning false for anything other than
non-negative integers.

Also clarified in docs.

PR-URL: https://github.com/nodejs/node/pull/15567


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>