Improved alpha_composite documentation

[radarhere]

When calling alpha_composite(im1, im2), both images must be of mode RGBA

• An error is thrown at https://github.com/python-pillow/Pillow/blob/master/libImaging/AlphaComposite.c#L36 if the mode of the first image is not RGBA.
• The modes of the two images must also match, or an error will be thrown at https://github.com/python-pillow/Pillow/blob/master/libImaging/AlphaComposite.c#L43

It is #1695 that points out that the documentation could be improved.