Add some extra warnings about the reference cycle dangers of `RegistryKey`

src/types.rs

@@ -23,6 +23,11 @@ pub struct LightUserData(pub *mut c_void);
 /// reference to a parent `Lua` instance, and thus is managed differently.  Though it is more
 /// difficult to use than the normal handle types, it is Send + Sync + 'static, which means that it
 /// can be used in many situations where it would be impossible to store a regular handle value.
+///
+/// Be warned, If you place this into Lua via a `UserData` type, it is *very easy* to accidentally
+/// cause reference cycles that the Lua garbage collector cannot resolve.  Instead of placing a
+/// `RegistryKey` into a `UserData` type, prefer instead to use `UserData::set_user_value` /
+/// `UserData::get_user_value`.
 pub struct RegistryKey {
     pub(crate) registry_id: c_int,
     pub(crate) unref_list: Arc<Mutex<Option<Vec<c_int>>>>,