doc/go1.12: add note about CL 153559's syscall/js.Callback rename

Updates #28711

Change-Id: I03139a394fdf0540db07d6d1e38b3fa223b06d58
Reviewed-on: https://go-review.googlesource.com/c/154059Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.12.html

@@ -617,6 +617,18 @@ for {
 
 <dl id="syscall/js"><dt><a href="/pkg/syscall/js/">syscall/js</a></dt>
   <dd>
+    <p><!-- CL 153559 -->
+    <p>
+      The <code>Callback</code> type and <code>NewCallback</code> function have been renamed;
+      they are now called
+      <a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#Func"><code>Func</code></a> and
+      <a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#FuncOf"><code>FuncOf</code></a>, respectively.
+      This is a breaking change, but WebAssembly support is still experimental
+      and not yet subject to the
+      <a href="/doc/go1compat">Go 1 compatibility promise</a>. Any code using the
+      old name will need to be updated.
+    </p>
+
     <p><!-- CL 141644 -->
       TODO: <a href="https://golang.org/cl/141644">https://golang.org/cl/141644</a>: add Wrapper interface to support external Value wrapper types
     </p>