Fix post process function called in the instance segmentation example of mask2former
Author: OnTheThirdDay
Pull request: yes

# What does this PR do?

- change doc of mask2former example of post_process_instance_segmentation to the right function call.
- change doc of maskformer and mask2former on post_process_instance_segmentation for clarity on working with overlapping instances.
- change examples in doc of segmentation models from image[::-1] to (image.height, image.width) for clarity.



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Comments:
This looks like a good fix to me, but cc @nielsrogge to confirm!

I just made another change to the doc of post_process_instance_segmentation of both maskformer and mask2former image processors.

I added this line, as current default behavior of the method will not deal with the cases where instances could overlap, but the descriptions related to `return_coco_annotation ` and `return_binary_maps`, though clearly understandable, are not very intuitive for users to realize they need to use them.

```
If instances could overlap, set either return_coco_annotation or return_binary_maps to `True` to get the correct segmentation result.
```

Additionally, I changed this line to make it more precise for the description of the returned result covering the case where `return_binary_maps` is set to `True`.

```
`List[Dict]`: A list of dictionaries, one per image, each dictionary containing two keys:
            - **segmentation** -- A tensor of shape `(height, width)` where each pixel represents a `segment_id`, or
              `List[List]` run-length encoding (RLE) of the segmentation map if return_coco_annotation is set to
              `True`, or a tensor of shape `(num_instances, height, width)` if return_binary_maps is set to `True`.
              Set to `None` if no mask if found above `threshold`.
```

the pr now has included above mentioned changes on doc.
@NielsRogge @qubvel @ArthurZucker