spec: clarify when constant slice indices must be in range

spec: clarify when constant slice indices must be in range

This documents the status quo for most implementations,
with one exception: gc generates a run-time error for
constant but out-of-range indices when slicing a constant
string. See issue 7200 for a detailed discussion.

LGTM=r
R=r, rsc, iant, ken
CC=golang-codereviews
https://golang.org/cl/72160044