builtin: improve docs for make slice
builtin: improve docs for make slice

Fixes #22764

Change-Id: I339d3bbb08983e0d69d69a1d00c4d15d0661762a
Reviewed-on: https://go-review.googlesource.com/78335
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

Cofyc/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,codestation/go,josharian/go,cloudflare/go,golang/go,nawawi/go,josharian/go,josharian/go,akutz/go,nawawi/go,josharian/go,codestation/go,cloudflare/go,Cofyc/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,codestation/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,codestation/go,codestation/go,golang/go,josharian/go,Cofyc/go,golang/go,akutz/go,cloudflare/go,CAFxX/go,akutz/go,cloudflare/go,akutz/go,golang/go,Cofyc/go,Cofyc/go,CAFxX/go,Cofyc/go,nawawi/go,cloudflare/go,CAFxX/go,josharian/go,Cofyc/go,nawawi/go,codestation/go,akutz/go,akutz/go,codestation/go,Cofyc/go,akutz/go,CAFxX/go,josharian/go,CAFxX/go,nawawi/go,codestation/go,josharian/go,christopher-henderson/Go