Documentation bugs relating to animation groups, image providers, Component, Transition

Some doc fixes needed:

• Animations within ParallelAnimation and SequentialAnimation can't be started/stopped individually
• QDeclarativeImageProvder::requestPixmap() has a note about reentrant calling, but it won't be called from outside the main thread
• Component main documentation's example snippet has no size set for the root Item
• Transition main documentation's example code specifies multiple Transition items in a list without comma separators