docs: describe the unregistered worker failure exactly

Review feedback on #66. The guide said that a message for an
unregistered actor class waits with no error. That was wrong. Rerun
against solid-objects 0.17.1 with an instrumentation callback, the
worker reported solid_objects.activation.failed six times in four
seconds with errorName UnknownActorType, and returned the message to
the queue each time without a counted attempt. Nothing printed
without the callback, which is why the first run looked silent.

The guide and the changelog now describe that failure. The step 7
note also points to the direct ref() call above instead of the step 6
class definition, which registers nothing.

Author: cardmagic

CHANGELOG.md

@@ -5,8 +5,11 @@
 - Correct `docs/agents.md`: `reject` takes a code and an options object,
   `this.reject(code, { message })`, not `this.reject(code, message)`.
 - `docs/agents.md` now says that a worker process runs only the actor classes
-  that it knows. A message for an unregistered class stays in SQL with no
-  attempt and no error, so register each class before `runtime.run(signal)`.
+  that it knows. For an unregistered class, actor setup fails with
+  `UnknownActorType`, the worker reports `solid_objects.activation.failed`,
+  and the message returns to the queue without a counted attempt. Nothing
+  prints without an `instrumentation` callback. Register each class before
+  `runtime.run(signal)`.
   It also tells agents to install the current release, puts the
   deny-by-default reminder beside the install step, and lists the API
   mistakes found in agent-written code.

docs/agents.md

@@ -222,9 +222,12 @@ runs in the caller. These features need a process that calls
 - Messages to other actors.
 - Realtime broadcasts.
 
-The worker process runs only the actor classes that it knows. Register each
-class before `run()`. A message for an unregistered class stays in SQL with no
-attempt and no error:
+The worker process runs only the actor classes that it knows. For a message
+whose class is not registered, actor setup fails with `UnknownActorType`. The
+worker reports `solid_objects.activation.failed` through instrumentation,
+returns the message to the queue without counting an attempt, and tries again.
+Nothing prints unless the application sets an `instrumentation` callback, so
+the message seems to wait with no error. Register each class before `run()`:
 
 ```typescript
 runtime.register(TicketSale)
@@ -236,7 +239,7 @@ await runtime.close()
 ```
 
 `TicketSale.ref(...)` also registers the class, so a script that calls `ref()`
-before `run()`, like the example in step 6, already works.
+before `run()`, like the direct call above, already works.
 
 The packaged `solid-objects start` command does the same for a runtime that
 `solid-objects.config.js` exports. When no process runs, committed work waits