The documentation for the `frame_change_pre` and `frame_change_post` handlers was really sparse, and suggested that `frame_change_pre` is called before the frame changes. This is not the case; it's called after the frame has changed, but before the dependency graph has been updated for it. No functional changes, just updated documentation. Differential Revision: https://developer.blender.org/D7905 Reviewed by: sergey