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>

lib/events.js

@@ -351,7 +351,7 @@ EventEmitter.prototype.prependOnceListener =
       return this;
     };
 
-// emits a 'removeListener' event iff the listener was removed
+// Emits a 'removeListener' event if and only if the listener was removed.
 EventEmitter.prototype.removeListener =
     function removeListener(type, listener) {
       var list, events, position, i, originalListener;