[api] Add documentation for PropertyCallbackInfo.This().

BUG=v8:5260

Review-Url: https://codereview.chromium.org/2278523002
Cr-Commit-Position: refs/heads/master@{#38949}

include/v8.h

@@ -3346,6 +3346,47 @@ class PropertyCallbackInfo {
    */
   V8_INLINE Local<Value> Data() const;
 
+  /**
+   * \return The receiver. In many cases, this is the object on which the
+   * property access was intercepted. When using
+   * `Reflect.Get`, `Function.prototype.call`, or similar functions, it is the
+   * object passed in as receiver or thisArg.
+   *
+   * \code
+   *  void GetterCallback(Local<Name> name,
+   *                      const v8::PropertyCallbackInfo<v8::Value>& info) {
+   *     auto context = info.GetIsolate()->GetCurrentContext();
+   *
+   *     v8::Local<v8::Value> a_this =
+   *         info.This()
+   *             ->GetRealNamedProperty(context, v8_str("a"))
+   *             .ToLocalChecked();
+   *     v8::Local<v8::Value> a_holder =
+   *         info.Holder()
+   *             ->GetRealNamedProperty(context, v8_str("a"))
+   *             .ToLocalChecked();
+   *
+   *    CHECK(v8_str("r")->Equals(context, a_this).FromJust());
+   *    CHECK(v8_str("obj")->Equals(context, a_holder).FromJust());
+   *
+   *    info.GetReturnValue().Set(name);
+   *  }
+   *
+   *  v8::Local<v8::FunctionTemplate> templ =
+   *  v8::FunctionTemplate::New(isolate);
+   *  templ->InstanceTemplate()->SetHandler(
+   *      v8::NamedPropertyHandlerConfiguration(GetterCallback));
+   *  LocalContext env;
+   *  env->Global()
+   *      ->Set(env.local(), v8_str("obj"), templ->GetFunction(env.local())
+   *                                           .ToLocalChecked()
+   *                                           ->NewInstance(env.local())
+   *                                           .ToLocalChecked())
+   *      .FromJust();
+   *
+   *  CompileRun("obj.a = 'obj'; var r = {a: 'r'}; Reflect.get(obj, 'x', r)");
+   * \endcode
+   */
   V8_INLINE Local<Object> This() const;
 
   /**