feat(rails-status): add JSDoc (#679)

src/rails-status.js

@@ -1,3 +1,4 @@
+/** Status order: later elements have higher precedence. */
 const statusOrder = [
   'deprecated',
   'experimental',
@@ -6,6 +7,12 @@ const statusOrder = [
   'stable'
 ]
 
+/**
+ * Get the latest status, based on the order of precedence
+ * 
+ * @param {readonly string[]} statuses list of status to pick from
+ * @returns {string | null} latest status if valid, `null` otherwise
+ */
 export const latestStatusFrom = (statuses) => {
   let highestRank = -1
   let latestStatus = null
@@ -22,6 +29,13 @@ export const latestStatusFrom = (statuses) => {
   return latestStatus
 }
 
+/**
+ * Compare function, which is compatible with #Array.sort.
+ * 
+ * @param {string} first
+ * @param {string} second
+ * @returns {1 | 0 | -1} sorting order
+ */
 export const compareStatuses = (first, second) => {
   const firstRank = statusOrder.indexOf(first)
   const secondRank = statusOrder.indexOf(second)