spec: clarify that newlines are kept in raw string literals

Fixes #8007.

LGTM=r
R=gri, r
CC=golang-codereviews
https://golang.org/cl/91510044

doc/go_spec.html

@@ -471,7 +471,7 @@ string composed of the uninterpreted (implicitly UTF-8-encoded) characters
 between the quotes;
 in particular, backslashes have no special meaning and the string may
 contain newlines.
-Carriage returns inside raw string literals
+Carriage return characters ('\r') inside raw string literals
 are discarded from the raw string value.
 </p>
 <p>