Explain how Image.set_pixel() works when the image format is not color

[Yobbolita]

Your Godot version:

4.5

Issue description:

The Image.set_pixel() function has the following signature :

void set_pixel(x: int, y: int, color: Color)

However, the image format can be of different types, including types like L8 with only luminance and no colors.

In that case, the pixel contains a single int, not a color. What do you put as the color parameter of set_pixel() then ?

After testing, it appears that you can still set a pixel's value on a L8 image using set_pixel, the R value provided will be used as the luminance value, while G and B will be ignored. (that's what seems to be after light testing)

I think this deserve to be better explained in the docs, as it does seem like a set_pixel_luminance() function is missing if you don't know how this function works.

URL to the documentation page (if already existing):

https://docs.godotengine.org/en/stable/classes/class_image.html#class-image-method-set-pixel