go_spec.html: clarify rune and string literals

go_spec.html: clarify rune and string literals
No changes to the meaning, just clearer language and more
examples, including illegal rune and string literals.
In particular, "character literal" and "character constant"
are now called "rune literal" and "rune constant" and the
word "character" always refers to the source text, not
program values.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/6448137