doc/go1.3.html: update for FreeBSD/ARM support.

FreeBSD 10 is fully supported.

LGTM=r
R=golang-codereviews, bradfitz, r
CC=dave, golang-codereviews
https://golang.org/cl/99810044

doc/go1.3.html

@@ -70,6 +70,11 @@ As of Go 1.3, support for Go on FreeBSD requires that the kernel be compiled wit
 <code>COMPAT_FREEBSD32</code> flag configured.
 </p>
 
+<p>
+In concert with the switch to EABI syscalls for ARM platforms, Go 1.3 will run only on
+FreeBSD 10. The x86 platforms, 386 and amd64, are unaffected.
+</p>
+
 <h2 id="memory">Changes to the memory model</h2>
 
 <p>