chore: Clarify the usage of agent_state

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 828733706

Author: DeanChensj

src/google/adk/events/event_actions.py

@@ -100,10 +100,11 @@ class EventActions(BaseModel):
   end_of_agent: Optional[bool] = None
   """If true, the current agent has finished its current run. Note that there
   can be multiple events with end_of_agent=True for the same agent within one
-  invocation when there is a loop."""
+  invocation when there is a loop. This should only be set by ADK workflow."""
 
   agent_state: Optional[dict[str, Any]] = None
-  """The agent state at the current event."""
+  """The agent state at the current event, used for checkpoint and resume. This
+  should only be set by ADK workflow."""
 
   rewind_before_invocation_id: Optional[str] = None
   """The invocation id to rewind to. This is only set for rewind event."""