spec: be clearer about definition of string length
spec: be clearer about definition of string length

Adjusted spec to explicitly define the string length as the
number of bytes of the string; the prose now matches the prose
for arrays. Made analogous change for slices.

Fixes #28736.

Change-Id: I47cab321c87de0a4c482f5466b819b2cc8993fd1
Reviewed-on: https://go-review.googlesource.com/c/149077
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,akutz/go,akutz/go,golang/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,josharian/go,josharian/go,CAFxX/go,josharian/go,golang/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,josharian/go,akutz/go,cloudflare/go,josharian/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,josharian/go,akutz/go,josharian/go,nawawi/go,akutz/go,golang/go,akutz/go,golang/go