docs(audience-diagnostics): comment the bare diagnostic getters

Addresses PR #708 review: adds per-getter comments for `Initialized`,
`UserId`, `SessionId` so all six public getters document their own
non-obvious semantics, not just the three that did before.

The block header above the getters still carries the shared contract
(zero-value default outside Init..Shutdown, tick-level snapshot); the
new lines only add what a reader cannot derive from the member name.

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

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/ImmutableAudience.cs

@@ -56,12 +56,15 @@ public static class ImmutableAudience
         // zero-value default (false / null / 0 / ConsentLevel.None) outside
         // Init..Shutdown. Values are tick-level snapshots, not invariants.
 
+        // Flipped true at the end of a successful Init, false by 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;
 
+        // Last value passed to Identify(). Cleared by Reset and any consent
+        // downgrade out of Full.
         public static string? UserId => _state.UserId;
 
         // Display-only — Reset and SetConsent(None) wipe it, so it is not
@@ -78,6 +81,7 @@ public static string? AnonymousId
             }
         }
 
+        // Changes on extended-pause rollover. Also null while consent is None.
         public static string? SessionId => _session?.SessionId;
 
         // Memory + disk counts are read without holding the drain lock, so