Add comment to get_kv_connector_kv_cache_events for clarity

Clarify that it is the workser side that calls the method.

Review comment:

- vllm-project#28309 (comment)

Signed-off-by: Martin Hickey <martin.hickey@ie.ibm.com>

Author: hickeyma

vllm/distributed/kv_transfer/kv_connector/v1/base.py

@@ -353,6 +353,8 @@ def get_kv_connector_stats(self) -> Optional["KVConnectorStats"]:
     def get_kv_connector_kv_cache_events(self) -> Optional["KVEventBatch"]:
         """
         Get the KV connector kv cache events collected during the last interval.
+        This function should be called by the model runner every time after the
+        model execution and before cleanup.
         """
         return None