fix: restore doc bar on RtcSession accessors; refresh stale README status

Two Important findings from the final whole-branch review:

T4-M3: RtcSession::channel_id/channel_state/is_closed lacked /// docs
(AGENTS.md mandates /// on every public item; learner-facing code is a
primary product). One-line docs added per existing /// style in rtc_session.rs.

T7-M2: README "Status" section still read "Scoping. No code yet" — stale
post-slice-1. Updated to reflect slice-1 implementation status; minimal
edit, surrounding content preserved.

crates/rutster-media/src/rtc_session.rs

@@ -151,14 +151,22 @@ impl RtcSession {
         })
     }
 
+    /// Returns the session's `ChannelId` — also the REST API session id
+    /// surfaced at `/v1/sessions/{id}` (spec §4.5).
     pub fn channel_id(&self) -> ChannelId {
         self.channel.id
     }
 
+    /// Returns the current signaling state (`ChannelState` from
+    /// `rutster-call-model`): `New` → `Connecting` → `Connected` →
+    /// `Closing` → `Closed`.
     pub fn channel_state(&self) -> ChannelState {
         self.channel.state
     }
 
+    /// True once the channel has reached `ChannelState::Closed` — str0m
+    /// resources are dropped; the loop driver returns `None` and the
+    /// tokio task tears down.
     pub fn is_closed(&self) -> bool {
         matches!(self.channel.state, ChannelState::Closed)
     }