doc: go1.12: document FreeBSD 12.0 requires COMPAT_FREEBSD11

Fixes #22447
Fixes #22448

Change-Id: Ia24f42c31e014c79040ff927f1247dfb2318de4f
Reviewed-on: https://go-review.googlesource.com/c/160778Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

doc/go1.12.html

@@ -47,6 +47,7 @@ Do not send CLs removing the interior tags from such phrases.
   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+.
+  FreeBSD 12.0+ requires a kernel with the COMPAT_FREEBSD11 option set (this is the default).
 </p>
 
 <p><!-- CL 146898 -->