qdoc: Broken links for functions in a namespace

If a module contains a documented function under a namespace, and that namespace originates from ( - is documented in) another module, the links to that function is broken in offline mode, which uses separate output sub-folders for each module docs.

Qt 5.6.0RC: In Assistant, open Qt Core -> Qt Namespace -> Functions. The functions are documented on the same page, but the links go to Qt GUI instead.