
document the special value '-1' in the granulepos field.

svn path=/trunk/ogg/; revision=1295

brion/ogg,mwgoldsmith/ogg,xiph/ogg,eidy/libogg,nicolaichuk/ogg,nicolaichuk/ogg,brion/ogg,ShiftMediaProject/ogg,xiph/ogg,ksophocleous/ogg,felipebetancur/ogg,youfearm3/ogg,emscripten-ports/Ogg,emscripten-ports/Ogg,felipebetancur/ogg,eidy/libogg,mwgoldsmith/ogg,ksophocleous/ogg,youfearm3/ogg,ShiftMediaProject/ogg