docs(discovery): precise wording on the watchdog, the empty state, recovery

Three comments this repair touches were still loose: "never rebuilt on a state
poll" (the discovery table, not the watchdog — the sensor's own poll does
refresh it when it has no fresh answer), and the class docstring's "recorded as
unknown" now says where a reader actually sees it, since the sensor reports
not_connected while the transport is down and unknown once it is up but the
sweep delivered nothing.

custom_components/nvidia_shield/connection.py

@@ -229,7 +229,7 @@ class ShieldConnectionManager:
         # here (t62 F1): a failed or unavailable sweep records itself as UNKNOWN
         # instead, because stamping an empty answer would serve "0 apps" — and
         # block every retry — for the whole APP_LIST_TTL. Refreshed on demand;
-        # never rebuilt on a state poll.
+        # the watchdog cycle never rebuilds it.
         self._installed_apps: tuple[list[dict], float] | None = None
         # t62 F1: True when the last sweep could not deliver an answer (no
         # poller, ADB down, a shell that raised or answered nothing) — so an