Since WordPress 3.5, `wp_save_image_file()` uses `WP_Image_Editor` classes under the hood to save the images.
While the `save()` method in those instances returns `array|WP_Error` and is documented as such, the return type of the `wp_save_image_file()` function was still left as `bool`.
A better solution would be to adjust the function to return the documented boolean type. However, doing that after 20+ major WP releases would be a backward compatibility break, so the documentation is now updated instead.
Includes:
* Documenting the returned array using hash notation.
* Adding a `@since` note for the `$image` parameter expecting a `WP_Image_Editor` instance.
* Adding a `@since` note for the `$filesize` value being included in the returned array.
Follow-up to [22094], [22619], [52837].
Props jrf, SergeyBiryukov.
See #55646.
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