webp-lossless-bitstream-spec: reword abstract re alpha
webp-lossless-bitstream-spec: reword abstract re alpha

'zero alpha pixels' was a bit hard to parse; replace this with something
more explicit

Bug: webp:448
Change-Id: Ifc8c93af5520ffdafc58e3fc311dfb4cb19626e9

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp