doc/articles: state that slices/arrays start at the 0th element
doc/articles: state that slices/arrays start at the 0th element

Fixes issue 5881.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/11892043

d0f/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,bryanxu/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,mhennings/marcohennings-go,rdp/rogerpack2005-golang,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,mhennings/marcohennings-go,webfd/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,d0f/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,bryanxu/go-zh