webp-container-spec: update note on trailing data
webp-container-spec: update note on trailing data

clarify that the data is after the size specified by the file size in
the header; an alternate way to read the previous statement was that the
data was after the 'WEBP' fourcc.

based on comments from:
https://datatracker.ietf.org/doc/draft-zern-webp/ballot/#draft-zern-webp_robert-wilton

Bug: webp:448
Change-Id: I7c5c5440a94cb817da51fe07d1ccf45d6af0f001

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