doc/go1.12: announce deprecation of support for FreeBSD 10.x

Fixes #27619

Change-Id: If18df696c0778efe894a4a249d4964db1b02e5d6
Reviewed-on: https://go-review.googlesource.com/c/150159Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.12.html

@@ -37,6 +37,16 @@ Do not send CLs removing the interior tags from such phrases.
   There are no changes to the language specification.
 </p>
 
+<h2 id="ports">Ports</h2>
+
+<h3 id="freebsd">FreeBSD</h3>
+
+<p>
+  Go 1.12 is the last release that is supported on FreeBSD 10.x, which has
+  already reached end-of-life. Go 1.13 will require FreeBSD 11.2+ or FreeBSD
+  12.0+.
+</p>
+
 <h2 id="tools">Tools</h2>
 
 <h3 id="gocache">Build cache requirement</h3>