The problem that if view documentation via web, for example:

http://doc.qt.io/qt-5/qpainter.html#drawText

documentation exists,

but if open documentation for `QPainter` inside assistant than there is no
documentation for `QPainter::drawText`, but for some other member functions of `QPainter`
documentation is possible to see.

If open local documentation in browser `xdg-open /usr/share/doc/qt/qtgui/qpainter.html`,
I see the same picture as in assistant.

So looks like for some reason script that generate /usr/share/doc/qt/qtgui/qpainter.html works properly for Qt web site, but fails for local.