document that img handle constructors guess fmt

This patch documents the fact that a couple of the image
handle constructors know how to guess the image format based
on the data that they are provided.

I had to dig through `iced` sources until I discovered that those
routines ultimately boil down to stuff like [image::load_from_memory][1]
from the `image` crate, so I thought I would save others the trouble
of doing the same reverse-engineering

[1]: https://docs.rs/image/0.23.4/image/fn.load_from_memory.html

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