chore(docs): add missing comments (#14003)

This is a load-bearing change to avoid duplicate licenses in closure-compiled bundles.
See https://github.com/angular/tsickle/issues/332

modules/@angular/core/src/util/lang.ts

@@ -6,6 +6,9 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+/**
+ * Determine if the argument is shaped like a Promise
+ */
 export function isPromise(obj: any): obj is Promise<any> {
   // allow any Promise/A+ compliant thenable.
   // It's up to the caller to ensure that obj.then conforms to the spec