QQuickWindow: add some links to resetOpenGLState

... or people might not find it. So link it from the docs of the
various signals which expect the users to leave the GL state as
they found it.

Change-Id: I1ae41958449ded110aa2398b415d593bf04d2b7f
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>