image_downsize doesn't document the return value

[rmccue]

Neither the filter nor function for image_downsize document fully the return value. This means that in order to write your own downsizer, you need to look at how it works internally, which is a pain.

[wonderboymusic]

I look forward to this

[DrewAPicture]

In 34362:

Docs: Add a more complete return description for the array of data (or false) returned by image_downsize().

If not short-circuited via the image_downsize boolean filter, image_downsize() returns an array of data including the image URL, width, height, and whether the image size is intermediate, in that order. Otherwise, it returns false.

Fixes #30636.