refactor: add missing jsdoc inside calculateRank.js (anuraghazra#3155)

Author: qwerty541

src/calculateRank.js

@@ -1,7 +1,19 @@
+/**
+ * Calculates the exponential cdf.
+ *
+ * @param {number} x The value.
+ * @returns {number} The exponential cdf.
+ */
 function exponential_cdf(x) {
   return 1 - 2 ** -x;
 }
 
+/**
+ * Calculates the log normal cdf.
+ *
+ * @param {number} x The value.
+ * @returns {number} The log normal cdf.
+ */
 function log_normal_cdf(x) {
   // approximation
   return x / (1 + x);