Ambiguous license notice in code files

Code files have this type of notice:
```
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
```
Taken alone such a notice is a tad ambiguous? do both the GPL AND the Apache license apply at the same time? or is this choice?

Of course the README makes this clear that this is a choice (e.g. OR, not AND):

```
License

RocksDB is dual-licensed under both the GPLv2 (found in the COPYING file in the root directory) and Apache 2.0 License (found in the LICENSE.Apache file in the root directory). You may select, at your option, one of the above-listed licenses.
```

It would be great to have clear source code-level notices.

This has been found by @royaljust