doc/go1.3.html: add syscall.SendmsgN

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/93430044

doc/go1.3.html

@@ -457,6 +457,14 @@ now considers the <code>DEL</code> character, <code>U+007F</code>, to be
 non-printing.
 </li>
 
+<li>
+The <a href="/pkg/syscall/"><code>syscall</code></a> package now provides
+<a href="/pkg/syscall/#SendmsgN"><code>SendmsgN</code></a>
+as an alternate version of
+<a href="/pkg/syscall/#Sendmsg"><code>Sendmsg</code></a>
+that returns the number of bytes written.
+</li>
+
 <li>
 On Windows, the <a href="/pkg/syscall/"><code>syscall</code></a> package now
 supports the cdecl calling convention through the addition of a new function