Add documentation for threadsafe option in FFI JSCallback (#15982)

Co-authored-by: Don Isaac <[email protected]>

docs/api/ffi.md

@@ -297,6 +297,20 @@ setTimeout(() => {
 
 When you're done with a JSCallback, you should call `close()` to free the memory.
 
+### Experimental thread-safe callbacks
+`JSCallback` has experimental support for thread-safe callbacks. This will be needed if you pass a callback function into a different thread from it's instantiation context. You can enable it with the optional `threadsafe` option flag.
+```ts
+const searchIterator = new JSCallback(
+  (ptr, length) => /hello/.test(new CString(ptr, length)),
+  {
+    returns: "bool",
+    args: ["ptr", "usize"],
+    threadsafe: true, // Optional. Defaults to `false`
+  },
+);
+```
+Be aware that there are still cases where this does not 100% work.
+
 {% callout %}
 
 **⚡️ Performance tip** — For a slight performance boost, directly pass `JSCallback.prototype.ptr` instead of the `JSCallback` object: