doc/go1.14: document freebsd/arm64 port

Updates #24715

Change-Id: Ie6421cf2d599740bbb0576bcc37bd8ddf5af3961
Reviewed-on: https://go-review.googlesource.com/c/go/+/210119Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.14.html

@@ -66,6 +66,13 @@ TODO
   64-bit <code>darwin/arm64</code> port.
 </p>
 
+<h3 id="freebsd">FreeBSD</h3>
+
+<p><!-- golang.org/issue/24715 -->
+  Go now supports the 64-bit ARM architecture on FreeBSD (the
+  <code>freebsd/arm64</code> port).
+</p>
+
 <h3 id="nacl">Native Client (NaCl)</h3>
 
 <p><!-- golang.org/issue/30439 -->