spec: clarify section on string types

spec: clarify section on string types

Strings happen to be represented similarly to
byte slices internally, but they don't quite
behave like them: While strings can be indexed,
sliced, and have their len() taken like byte
slices, string elements are not addressable,
make() and cap() is not supported, range loops
operate differently, and they are immutable (and
thus behave like values rather then references).

Fixes #4018.

R=r, rsc, iant, ken
CC=golang-dev
https://golang.org/cl/6503116