Comment # 3 on bug 1162786 from Fabian Vogt

Urgh:

> WARNING: Python version 2 (2.7.5 or later) is required to build QtWebEngine.

It might be useful to build the WE docs as part of libqt5-qtwebengine instead
of libqt5-qtdoc to avoid having to add all the WE BRs to qtdoc as well.