Consistently use @returns for returned data types in JSDoc comments

Sometimes we also used `@return`, but `@returns` is what the JSDoc
documentation recommends. Even though `@return` works as an alias, it's
good to use the recommended syntax and to be consistent within the
project.

src/shared/util.js

@@ -820,7 +820,7 @@ function isSpace(ch) {
  * Creates a promise capability object.
  * @alias createPromiseCapability
  *
- * @return {PromiseCapability}
+ * @returns {PromiseCapability}
  */
 function createPromiseCapability() {
   const capability = Object.create(null);