Fix missing QQuickTextDocument docs

It didn't appear in the docs at all. Moving the documentation to .cpp
fixes the problem.

Change-Id: Id9741bc6dab20ba976952143160d3551787fae40
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>