Blender Git Loki

Fix T77126: Documented frame_change_pre and frame_change_post handlers

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