Fix post_process_object_detection method descriptions
# What does this PR do?
Fixes the descriptions of all effected model methods (post_process_object_detection and deprecated post_process methods) that inaccurately state the methods returns bounding boxes in the format expected by the COCO API (x_center, y_center, w, h) format instead of the (x1, y1, x2, y2) format. 

I will open a separate PR to add an option to return the bounding boxes in the COCO API format. 


## Before submitting
- [ X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).


_The documentation is not available anymore as the PR was closed or merged._