Change _update_datatype_callbacks to _on_update_datatype_callbacks

[GDYendell]

This should be updated to match the the new naming of similar methods. These callbacks are called as a side effect of the datatype being changed (like on_update), not to action changing the datatype (like update).

Document breaking changes - I think the method to register a callback will have to change.

Acceptance Criteria

• Member variable and all surrounding names updated
• Docs updated