At the moment, gc and gccgo report compile-
time errors for certain constant indexes that
are out of bounds. The spec however requests
a run-time panic for out-of-bounds indexes
(http://tip.golang.org/ref/spec#Indexes).

Document the status quo.

Fixes #4231.

R=r, rsc, iant, ken
CC=golang-dev
https://golang.org/cl/6699048