Documentation of internal image format compatibilty

**Your Godot version:**

3.2.1

**Issue description:**

It would be great to enhance the documentation around `ImageFormat`/`Image`/`Texture` to give rough hints about what compatibility guarantees Godot provides. Currently the documentation doesn't touch:

1. The information that support for certain image formats is platform dependent.
2. Rough guidelines which formats are supported broadly, and which formats are known to be missing on certain platforms.
3. If a platform does not support a format where exactly does it fail, i.e., how to write fail-safe image/texture loading code. For instance: Will even creating an `Image` fail, or will the failure happen only when binding it to a texture?

In my case the lack of this information made me conclude that Godot is internally using some kind of shims on platforms that don't support certain formats, giving the impression that it is safe to use any format. I've now invested in a design that probably doesn't work at least on iOS as [indicated by this issue](https://github.com/godotengine/godot/issues/25325#issuecomment-630308109).

**URL to the documentation page (if already existing):**

These were the places where I was looking for such information:

- [`Image`](https://docs.godotengine.org/en/3.2/classes/class_image.html#image)
- [`ImageTexture`](https://docs.godotengine.org/en/3.2/classes/class_imagetexture.html#class-imagetexture)
- [`Texture`](https://docs.godotengine.org/en/3.2/classes/class_texture.html#class-texture)

Maybe I have just missed it somewhere else?