Add compat flag docs for FinalizationRegistry and WeakRef

src/content/compatibility-flags/js-weak-refs.md

@@ -0,0 +1,24 @@
+---
+_build:
+  publishResources: false
+  render: never
+  list: never
+
+name: "Enable `FinalizationRegistry` and `WeakRef`"
+sort_date: "2025-05-05"
+enable_date: "2025-05-05"
+enable_flag: "enable_weak_ref"
+disable_flag: "disable_weak_ref"
+---
+
+Enables the use of [`FinalizationRegistry`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry) and [`WeakRef`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakRef) built-ins.
+* `FinalizationRegistry` allows you to register a cleanup callback that runs after an object has been garbage-collected.
+* `WeakRef` creates a weak reference to an object, allowing it to be garbage-collected if no other strong references exist.
+
+:::note[Behaviour]
+`FinalizationRegistry` cleanup callbacks may execute at any point during your request lifecycle, even after your invoked handler has completed (similar to `ctx.waitUntil()`). These callbacks do not have an associated async context — you cannot perform any I/O within them, including emitting events to a tail Worker.
+:::
+
+:::caution
+These APIs are fundamentally non-deterministic — the timing and execution of garbage collection are unpredictable, and you **should not rely on them for essential program logic**. Additionally, cleanup callbacks registered with `FinalizationRegistry` may **never be executed**, including but not limited to cases where garbage collection is not triggered, or your Worker gets evicted.
+:::