lib: "iff" changed to "if and only if"

PR-URL: https://github.com/nodejs/node/pull/13496


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>