js: document utils.ajax()

app/javascript/shared/utils.js

@@ -50,6 +50,13 @@ export function delegate(eventNames, selector, callback) {
     );
 }
 
+// A promise-based wrapper for Rails.ajax().
+//
+// Returns a Promise that is either:
+// - resolved in case of a 20* HTTP response code,
+// - rejected with an Error object otherwise.
+//
+// See Rails.ajax() code for more details.
 export function ajax(options) {
   return new Promise((resolve, reject) => {
     Object.assign(options, {