doc/go1.16: mention syscall.AllThreadsSyscall

For #1435
For #40700

Change-Id: I01d277617ab511c90b9663fc89e418402e5ee2be
Reviewed-on: https://go-review.googlesource.com/c/go/+/285597
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.16.html

@@ -934,6 +934,16 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       and related calls are now implemented.
       Previously, they returned an <code>syscall.EOPNOTSUPP</code> error.
     </p>
+
+    <p><!-- CL 210639 -->
+      On Linux, the new functions
+      <a href="/pkg/syscall/#AllThreadsSyscall"><code>AllThreadsSyscall</code></a>
+      and <a href="/pkg/syscall/#AllThreadsSyscall6"><code>AllThreadsSyscall6</code></a>
+      may be used to make a system call on all Go threads in the process.
+      These functions may only be used by programs that do not use cgo;
+      if a program uses cgo, they will always return
+      <a href="/pkg/syscall/#ENOTSUP"><code>syscall.ENOTSUP</code></a>.
+    </p>
   </dd>
 </dl><!-- syscall -->