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 <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

CAFxX/go,cloudflare/go,ImJasonH/go,CAFxX/go,derekmarcotte/go,derekmarcotte/go,vsekhar/elastic-go,derekmarcotte/go,momchil-velikov/go,momchil-velikov/go,codestation/go,derekmarcotte/go,CAFxX/go,linux-on-ibm-z/go,nawawi/go,suncycheng/go,vsekhar/elastic-go,goodlang/good,josharian/go,christopher-henderson/Go,codestation/go,nawawi/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,akutz/go,josharian/go,goodlang/good,Cofyc/go,golang/go,ImJasonH/go,golang/go,golang/go,CAFxX/go,cloudflare/go,polyverse-security/go,suncycheng/go,Cofyc/go,goodlang/good,akutz/go,golang/go,polyverse-security/go,suncycheng/go,ImJasonH/go,goodlang/good,cloudflare/go,derekmarcotte/go,golang/go,vsekhar/elastic-go,suncycheng/go,codestation/go,momchil-velikov/go,suncycheng/go,linux-on-ibm-z/go,goodlang/good,ImJasonH/go,josharian/go,derekmarcotte/go,cloudflare/go,nawawi/go,cloudflare/go,Cofyc/go,polyverse-security/go,ImJasonH/go,linux-on-ibm-z/go,goodlang/good,goodlang/good,akutz/go,vsekhar/elastic-go,nawawi/go,Cofyc/go,linux-on-ibm-z/go,momchil-velikov/go,cloudflare/go,Cofyc/go,josharian/go,CAFxX/go,josharian/go,josharian/go,CAFxX/go,ImJasonH/go,Cofyc/go,polyverse-security/go,momchil-velikov/go,vsekhar/elastic-go,Cofyc/go,linux-on-ibm-z/go,momchil-velikov/go,nawawi/go,derekmarcotte/go,akutz/go,nawawi/go,akutz/go,ImJasonH/go,polyverse-security/go,cloudflare/go,golang/go,nawawi/go,vsekhar/elastic-go,suncycheng/go,golang/go,linux-on-ibm-z/go,polyverse-security/go,derekmarcotte/go,codestation/go,suncycheng/go,suncycheng/go,polyverse-security/go,josharian/go,CAFxX/go,goodlang/good,momchil-velikov/go,polyverse-security/go,linux-on-ibm-z/go,josharian/go,cloudflare/go,linux-on-ibm-z/go,vsekhar/elastic-go,ImJasonH/go,codestation/go,vsekhar/elastic-go,codestation/go,codestation/go,golang/go,Cofyc/go,akutz/go,codestation/go,momchil-velikov/go