docs: improve description of task_instance_key_str

Author: kevinhongzl

airflow-core/docs/templates-ref.rst

@@ -70,8 +70,9 @@ Variable                                    Type                  Description
 ``{{ var.value }}``                                               Airflow variables. See `Airflow Variables in Templates`_ below.
 ``{{ var.json }}``                                                Airflow variables. See `Airflow Variables in Templates`_ below.
 ``{{ conn }}``                                                    Airflow connections. See `Airflow Connections in Templates`_ below.
-``{{ task_instance_key_str }}``             str                   | A unique, human-readable key to the task instance. The format is
-                                                                  | ``{dag_id}__{task_id}__{ds_nodash}``.
+``{{ task_instance_key_str }}``             str                   | A human-readable key for the task instance,
+                                                                  | comprising the DAG id, task id, and the logical date of the run.
+                                                                  | The format is ``{dag_id}__{task_id}__{ds_nodash}``.
 ``{{ run_id }}``                            str                   The currently running :class:`~airflow.models.dagrun.DagRun` run ID.
 ``{{ dag_run }}``                           DagRun                The currently running :class:`~airflow.models.dagrun.DagRun`.
 ``{{ test_mode }}``                         bool                  Whether the task instance was run by the ``airflow test`` CLI.