doc: add go1.5 note about change to zero-sized fields in cgo

This documents the change made in https://golang.org/cl/12864 for
https://golang.org/issue/11925.

Update #11925.

Change-Id: Id09f2a489ea947a725ed12c9cf793e5daef07a06
Reviewed-on: https://go-review.googlesource.com/12866Reviewed-by: David Crawshaw <crawshaw@golang.org>

doc/go1.5.html

@@ -598,6 +598,12 @@ invalid when the current working directory changes.
 On Windows, cgo now uses external linking by default.
 </p>
 
+<p>
+When a C struct ends with a zero-sized field, but the struct itself is
+not zero-sized, Go code can no longer refer to the zero-sized field.
+Any such references will have to be rewritten.
+</p>
+
 <h2 id="performance">Performance</h2>
 
 <p>