docs(audience): comment Initialized, UserId, SessionId getters

- `Initialized`: true once Init() finishes; false before Init / after Shutdown.
- `UserId`: cleared by Reset and any consent downgrade out of Full.
- `SessionId`: new ID at Init, Reset, on resume from a long-enough background pause, and on granting consent from None.

Plain-language style matches the audience-diagnostics docs follow-up.

Addresses PR #709 review: three getters lacked the per-member docs that the other public diagnostics getters already carry.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/ImmutableAudience.cs

@@ -56,12 +56,18 @@ public static class ImmutableAudience
         // zero-value default (false / null / 0 / ConsentLevel.None) outside
         // Init..Shutdown. Values are tick-level snapshots, not invariants.
 
+        // True once Init() has finished setting things up. False before
+        // Init() and after Shutdown().
         public static bool Initialized => _initialized;
 
         // Persisted value, which may differ from AudienceConfig.Consent if
         // a prior session changed it.
         public static ConsentLevel CurrentConsent => _state.Level;
 
+        // The user ID from the most recent Identify() call. Becomes null
+        // after Reset(), and also when consent drops from Full down to
+        // Anonymous or None (since anonymous tracking must not know who
+        // the user is).
         public static string? UserId => _state.UserId;
 
         // Display-only — Reset and SetConsent(None) wipe it, so it is not
@@ -78,6 +84,11 @@ public static string? AnonymousId
             }
         }
 
+        // The current session's ID. A new ID is assigned each time a
+        // session starts: at Init(), at Reset(), and when the app resumes
+        // from the background after the previous session has timed out.
+        // Null while consent is None — granting consent starts a fresh
+        // session.
         public static string? SessionId => _session?.SessionId;
 
         // Memory + disk counts are read without holding the drain lock, so