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

pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang