go spec: minor fix, to match conversion rules 4 and 5
R=rsc
CC=golang-dev
https://golang.org/cl/1270042
diff --git a/doc/go_spec.html b/doc/go_spec.html
index 8c958b1..d56efcc 100644
--- a/doc/go_spec.html
+++ b/doc/go_spec.html
@@ -3278,7 +3278,8 @@
</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>
