doc/go1.16: document that on OpenBSD syscalls are now made through libc

Updates #36435, #40700.

Change-Id: I1e2ded111ad58066cc9f2c9d00e719497b0f34d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/287634
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>

doc/go1.16.html

@@ -80,6 +80,16 @@ <h3 id="openbsd">OpenBSD</h3>
   support cgo.
 </p>
 
+<p><!-- golang.org/issue/36435, many CLs -->
+  On the 64-bit x86 and 64-bit ARM architectures on OpenBSD (the
+  <code>openbsd/amd64</code> and <code>openbsd/arm64</code> ports), system
+  calls are now made through <code>libc</code>, instead of directly using
+  the <code>SYSCALL</code>/<code>SVC</code> instruction. This ensures
+  forward-compatibility with future versions of OpenBSD. In particular,
+  OpenBSD 6.9 onwards will require system calls to be made through
+  <code>libc</code> for non-static Go binaries.
+</p>
+
 <h3 id="386">386</h3>
 
 <p><!-- golang.org/issue/40255, golang.org/issue/41848, CL 258957, and CL 260017 -->