Clarify exif-copyrighted-false means only Copyright flag not set

The previous "Public domain" is misleading, as we can't assume
such a copyright status when the EXIF info is not set.
<https://translatewiki.net/wiki/Thread:Support/Change_MediaWiki:Exif-copyrighted-false>

Change-Id: Ic471ed5a997b476fd1e8b4a15106ffc8cd895197
Merge "Clarify exif-copyrighted-false means only Copyright flag not set"