Yep, it is documented.
See http://www.apache.org/dev/apply-license.html
"Yes. All source files should include the copyright notice boilerplate that indicates it is owned by the ASF and that it is distributed under the Apache License, Version 2.0."
In recent discussions on commons-dev, I asked specifically about build files and xdocs etc, and Craig McClanahan came back with the response that all source files needed a license, and the only exception that was offered as an example was the MANIFEST.MF (which has no comment facility). Another would be test data that doesn't include comments.
Done on core and plugins