Update exemplar.js

Added stub file's comments for task #3

Author: quintuple-mallard

exercises/concept/captains-log/.meta/exemplar.js

@@ -9,6 +9,7 @@
 export function randomShipRegistryNumber() {
   return "NCC-"+Math.floor(1000 + Math.random()*9000)
 }
+
 /**
  * Generates a random stardate.
  *
@@ -17,6 +18,12 @@ export function randomShipRegistryNumber() {
 export function randomStardate() {
   return 41000 + Math.random() * 1000
 }
+
+/**
+ * Generates a random planet class.
+ *
+ * @returns {string} a one-letter planet class.
+ */
 export function randomPlanetClass() {
   const planetClasses = ['D', 'H', 'J', 'K', 'L', 'M', 'N', 'R', 'T', 'Y']
   return planetClasses[Math.floor(Math.random() * 10)]