message: Revise a comment to reflect the new approach.
diff: @@ -154,20 +154,17 @@ private:
  mutable std::unique_ptr<_impl::SyncClient> m_sync_client;
  - // Protects m_sessions
  - mutable std::mutex m_session_mutex;
  -
  // Protects m_file_manager and m_metadata_manager
  mutable std::mutex m_file_system_mutex;
  std::unique_ptr<SyncFileManager> m_file_manager;
  std::unique_ptr<SyncMetadataManager> m_metadata_manager;
  - // Active sessions are sessions which the client code holds a strong
  - // reference to. When the last strong reference is released, the session is
  - // moved to inactive sessions. Inactive sessions are promoted back to active
  - // sessions until the session itself calls unregister_session to remove
  - // itself from inactive sessions once it's done with whatever async cleanup
  - // it needs to do.
  + // Protects m_sessions
  + mutable std::mutex m_session_mutex;
  +
  + // Map of sessions by path name.
  + // Sessions remove themselves from this map by calling `unregister_session` once they're
  + // inactive and have performed any necessary cleanup work.
  std::unordered_map<std::string, std::shared_ptr<SyncSession>> m_sessions;
  };
  