doc/go1.7.html: mention recent changes to Rand.Read

Updates #16124

Change-Id: Ib58f2bb37fd1559efc512a2e3cba976f09b939a0
Reviewed-on: https://go-review.googlesource.com/24520Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go1.7.html

@@ -782,6 +782,20 @@ package.
 </dd>
 </dl>
 
+<dl id="math_rand"><dt><a href="/pkg/math/rand/">math/rand</a></dt>
+
+<dd>
+<p>
+The
+<a href="/pkg/math/rand/#Read"><code>Read</code></a> function and
+<a href="/pkg/math/rand/#Rand"><code>Rand</code></a>'s
+<a href="/pkg/math/rand/#Rand.Read"><code>Read</code></a> method
+now produce a pseudo-random stream of bytes that is consistent and not
+dependent on the size of the input buffer.
+</p>
+</dd>
+</dl>
+
 <dl id="mime_multipart"><dt><a href="/pkg/mime/multipart/">mime/multipart</a></dt>
 
 <dd>