Re-order and better explain the new methods

For each triplet, explain in order of:
  * Generic: TypeDesc + image_span<byte> (full explanation)
  * implied type: image_span<T> (brief explanation)
  * implied type + contiguous: span<T> (brief explanation)

Also, move write_image FIRST, followed by write_scanline(s) and
write_tile(s), since the whole image will be the most common use case.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz