Add note to QQuickItem docs about directly setting properties in C++.

It's not obvious that calling setPosition() directly, for example,
won't trigger animations defined in behaviors that react to that
item's x and y properties changing, because a quick glance at the
code shows that they have almost identical code paths and both
emit geometryChanged().

Change-Id: Ic7f7019b1bbba24732569c0de777ee02d79c1d3b
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>