Information on "Premult Alpha" is vague

**Your Godot version:**
3.4.3.Stable

**Issue description:**
I think this section is pretty vague. I understand the concept of pre-multiplying alpha after looking it up, but what it actually does in Godot is not at all clear. Should this only be used on 2D? It mentions that this can also fix border pixels, but I don't see why that would be the case.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/assets_pipeline/importing_images.html#premultiplied-alpha