go spec: minor fix, to match conversion rules 4 and 5

R=rsc
CC=golang-dev
https://golang.org/cl/1270042

doc/go_spec.html

@@ -3278,7 +3278,8 @@ of <code>x</code> and thus has no run-time cost.
 </p>
 
 <p>
-Specific rules apply to conversions where <code>T</code> is a numeric or string type.
+Specific rules apply to conversions where <code>T</code> is a
+numeric or string type, or where <code>x</code> is of string type.
 These conversions may change the representation of a value and incur a run-time cost.
 </p>