Image channel is used when converting PA with an RGBA palette

[radarhere]

Resolves #6658

This PR documents that when converting a PA image with an RGBA palette, the alpha values from the image are used, rather than the values from the palette.

from PIL import Image, ImagePalette
im = Image.new("PA", (1, 2))
p = ImagePalette.ImagePalette("RGBA")
first = p.getcolor((1, 2, 3, 4))
second = p.getcolor((5, 6, 7, 8))
im.putpalette(p)
im.putpixel((0, 0), (first, 100))
im.putpixel((0, 1), (second, 200))

im_rgba = im.convert("RGBA")
print("(0, 0):", im_rgba.load()[0, 0])
print("(0, 1):", im_rgba.load()[0, 1])

gives

(0, 0): (1, 2, 3, 100)
(0, 1): (5, 6, 7, 200)