- Fix docs that say opacity changes can trigger an add transition since
  this is no longer the case
- Be more clear about when the move transition is applied
- Be more clear about which anchors are allowed/disallowed in which positioners
- Add example to Positioner attached property

Change-Id: I78417ac5b8eba1af5587955121b7fd4e2ca21169
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>