doc: QWidget::setEnabled cannot enable children of a disabled widget

The current documentation is not terribly clear on this topic, and there's a
couple of posts on various forums where people want to do this. In fact, the old
wording suggested (at least to me) that it is OK to explicitly override a
disabled state, which is apparently not true.

Change-Id: I10c54e0089e9ba5d16958aea62df27feafdf7b3d
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>