docs: correct Session SAFETY comment after crossbeam swap

`crossbeam_channel::Receiver<T>` is `Sync` (unlike `mpsc::Receiver`),
so the old "Receiver is `Send` but `!Sync`" justification no longer
matched the truth — `Session` is `!Sync` because of its `NonNull<…>`
field, not its receiver. Rewrite the SAFETY comment accordingly.

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

Author: pranc1ngpegasus

crates/wisp-audiokit/src/lib.rs

@@ -88,8 +88,9 @@ mod imp {
 
     // SAFETY: Session owns the C handle and the receiver. The handle is
     // an opaque pointer we never deref ourselves; the C side serializes
-    // access internally. The receiver is `Send` but `!Sync`, which matches
-    // the semantics we expose.
+    // access internally, so it is sound to move the handle across threads.
+    // (`Session` stays `!Sync` overall because the `NonNull` field is
+    // `!Sync` — only `Send` needs the manual impl.)
     unsafe impl Send for Session {}
 
     // Swift may invoke `on_result_thunk` / `on_log_thunk` from different