spec: clarify effect of rounding of constant expressions

Not a language change.

Fixes #11350.

Change-Id: I9b905f17d1ef2722cab4bae38a037270165c7d95
Reviewed-on: https://go-review.googlesource.com/11369Reviewed-by: Rob Pike <r@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of June 11, 2015",
+	"Subtitle": "Version of June 23, 2015",
 	"Path": "/ref/spec"
 }-->
 
@@ -4083,7 +4083,7 @@ the implementation restriction in the section
 on <a href="#Constants">constants</a>.  This rounding may cause a
 floating-point constant expression to be invalid in an integer
 context, even if it would be integral when calculated using infinite
-precision.
+precision, and vice versa.
 </p>