Clarify trajectory grouping

README.md

@@ -53,7 +53,7 @@ decode them with `json.loads`.
 | `trajectory_step_added` | `step_index`, `step` (`thought`, `code`, `output`, `error`, `reward`, `misc`) |
 | `trajectory_finished` | `reward`, `finish_message`, `error_message` |
 
-To reconstruct a trajectory, group events by `trajectory_id`. The `task` in `trajectory_task_set` is the
+Within a row, `events` interleaves events from the root agent and all of its sub-agents in time order. To get one agent's trajectory, filter by `trajectory_id`; each sub-agent's `parent_info` (in its `trajectory_created` event) links it to the agent that spawned it. Single-agent datasets have one trajectory per row. The `task` in `trajectory_task_set` is the
 state at the start of the run (for TextCraft, `task.misc.initial_inventory` is the true starting inventory).
 
 ## Viewing with platoon's visualization tool