Fix ctx.signal documentation (#260)

## Why

It didn't make sense, and wasn't clear.

## What changed

Update docs

Maybe worth considering renaming to "invocationFinishedSignal" or
something more verbose, as signals in JavaScript are typically used in
context of cancellation.

## Versioning

- [ ] Breaking protocol change
- [ ] Breaking ts/js API change

<!-- Kind reminder to add tests and updated documentation if needed -->

router/context.ts

@@ -73,7 +73,12 @@ export type ProcedureHandlerContext<State> = ServiceContext & {
   /**
    * This signal is a standard [AbortSignal](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal)
    * triggered when the procedure invocation is done. This signal tracks the invocation/request finishing
-   * for _any_ reason including the procedure invocation.
+   * for _any_ reason, for example:
+   * - client explicit cancellation
+   * - procedure handler explicit cancellation via {@link cancel}
+   * - client session disconnect
+   * - server cancellation due to client invalid payload
+   * - invocation finishes cleanly, this depends on the type of the procedure (i.e. rpc handler return, or in a stream after the client-side has closed the request writable and the server-side has closed the response writable)
    *
    * You can use this to pass it on to asynchronous operations (such as fetch).
    *