[bug]: inpainting docs need update regarding transparency

[keturn]

The inpainting docs make a big point of the importance of retaining the color values of transparent pixels: https://invoke-ai.github.io/InvokeAI/features/INPAINTING/

but the way things are now (2.2.3), any transparency in the input image gets filled in by patchmatch, which does its best to make it look like the stuff in that area was never there. That's great if you want to erase things, but it means it's no longer appropriate to use the alpha channel if you want inpainting to replace things. (Like a crow for a parrot.)

We can still replace things, but it requires passing a --mask for it, not erasing it in the alpha channel of the input image.