message: update with links to implementation
diff: * Author: nikomatsakis
  * Date: 2021-05-15
  -* Introduced in: N/A
  +* Introduced in: [salsa-rs/salsa#265](https://github.com/salsa-rs/salsa/pull/265)
  ## Summary
  @@ -34,12 +34,12 @@ When you do a write to the salsa database, that write will block until any queri
  The changes required to implement this RFC are as follows:
  -* Adjust comments on `is_current_revision_canceled`.
  -* Introduce a sentinel type/value `SalsaCancelationToken` that can be used with [`resume_unwind`](https://doc.rust-lang.org/std/panic/fn.resume_unwind.html)
  -* Introduce a `unwind_if_canceled` method into the runtime which checks whether cancelation has occured and panics if so.
  +* Remove on `is_current_revision_canceled`.
  +* Introduce a sentinel cancellation token that can be used with [`resume_unwind`](https://doc.rust-lang.org/std/panic/fn.resume_unwind.html)
  +* Introduce a `unwind_if_canceled` method into the `Database` which checks whether cancelation has occured and panics if so.
  + * This method also triggers a `salsa_event` callback.
  * This should probably be inline for the `if` with an outlined function to do the actual panic.
  * Modify the code for the various queries to invoke `unwind_if_canceled` when they are invoked or validated.
  - * QUESTION: Do we want to just check *every query access*, including cache hits? Probably. Is there a convenient time to do this in the runtime
  ## Frequently asked questions
  