spec: clarify acceptable indices in array/slice composite literals

spec: clarify acceptable indices in array/slice composite literals

This simply documents the status quo accepted by cmd/compile, gccgo,
and go/types. The new language matches the language used for indices
of index expressions for arrays and slices.

Fixes #16679.

Change-Id: I65447889fbda9d222f2a9e6c10334d1b38c555f0
Reviewed-on: https://go-review.googlesource.com/30474
Reviewed-by: Rob Pike <r@golang.org>