doc/go1.11: mention the libSystem.so change for iOS

The change to make the runtime use libSystem.so macOS instead of
direct kernel calls applies to iOS as well.

Change-Id: I97ea86452ac5f7433aea58bbd3ff53a2eb2835e0
Reviewed-on: https://go-review.googlesource.com/124657Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.11.html

@@ -307,9 +307,9 @@ func f(v interface{}) {
 </p>
 
 <p><!-- CL 108679 -->
-  On macOS, the runtime now uses <code>libSystem.so</code> instead of
+  On macOS and iOS, the runtime now uses <code>libSystem.so</code> instead of
   calling the kernel directly. This should make Go binaries more
-  compatible with future versions of macOS.
+  compatible with future versions of macOS and iOS.
   The <a href="/pkg/syscall">syscall</a> package still makes direct
   system calls; fixing this is planned for a future release.
 </p>