It does not change the license, but they are alerting you to what you are agreeing to by contributing.
What must be included, under the Apache 2 license, is something similar to this:
Copyright [yyyy] [name of copyright owner]
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
The license does not say where you have to include that, but you must give notice that you are using the Apache 2 license.
They are giving you the right to opt out by saying "unless you explicitly state otherwise".