doc: note that Go 1.9 is last release to support FreeBSD 9.3

Fixes #20930

Change-Id: I4a59de0556cffeae9af2eaa41609601e086211b2
Reviewed-on: https://go-review.googlesource.com/47731Reviewed-by: Rob Pike <r@golang.org>

doc/go1.9.html

@@ -99,6 +99,15 @@ type T1 = T2
   hardware.
 <p>
 
+<h3 id="freebsd">FreeBSD</h3>
+
+<p>
+  Go 1.9 is the last release that will run on FreeBSD 9.3,
+  which is already
+  <a href="https://www.freebsd.org/security/unsupported.html">unsupported by FreeBSD</a>.
+  Go 1.10 will require FreeBSD 10.3+.
+</p>
+
 <h3 id="openbsd">OpenBSD 6.0</h3>
 
 <p> <!-- CL 40331 -->