doc: add missing Selection type

Author: coolaj86

dashgov.js

@@ -320,10 +320,17 @@ DashGov.estimateBlockHeight = function (ms, secondsPerBlock) {
   return height;
 };
 
+/**
+ * @typedef Selection
+ * @prop {Estimate} start
+ * @prop {Estimate} end
+ */
+
 /**
  * @param {Estimates} estimates
  * @param {Uint32} startPeriod
  * @param {Uint32} endPeriod
+ * @returns {Selection}
  */
 DashGov.selectEstimates = function (estimates, startPeriod, endPeriod) {
   let startEstimate;