docs(snapshots): name the host-side observe-only refusal

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: thymikee

website/docs/docs/snapshots.md

@@ -54,7 +54,9 @@ left exactly as it was. Other platforms, provider-owned sessions, and `--diff` a
 
 The runner refuses with `OBSERVATION_UNAVAILABLE` (also named by `details.runnerErrorCode`) unless
 the app reports `runningForeground` before and after the capture under one process identity. It does
-not substitute SpringBoard or a presented system surface. Successful JSON includes:
+not substitute SpringBoard or a presented system surface. The host refuses with `COMMAND_FAILED` and
+`details.reason: "observation-unavailable"` when a runner answers an observe-only request without
+`observation` or with a `targetActivation` fact. Successful JSON includes:
 
 ```json
 {