Clarify what down-state means in docstrings

src/zino/statemodels.py

@@ -335,7 +335,10 @@ def dump_event_to_file(self, dir_name: str):
             statefile.write(self.model_dump_json(exclude_none=True, indent=2))
 
     def is_down(self) -> bool:
-        """Returns true if the event is in a down state"""
+        """Returns true if the event is in a down state.
+        What is considered a "down state" depends on the type of event,
+        but generally it means something like a device or a port is down.
+        """
         return False
 
 
@@ -358,6 +361,10 @@ def subindex(self) -> SubIndex:
         return self.ifindex
 
     def is_down(self) -> bool:
+        """Returns true if the event is in a down state.
+        A PortStateEvent is considered "down" if the port it is related to is
+        either down or in a flapping state.
+        """
         return self.portstate in [InterfaceState.DOWN, InterfaceState.FLAPPING]
 
 
@@ -399,6 +406,10 @@ class ReachabilityEvent(Event):
     reachability: Optional[ReachabilityState] = None
 
     def is_down(self) -> bool:
+        """Returns true if the event is in a down state.
+        A ReachabilityEvent is considered "down" if the device it is related to
+        is not reachable.
+        """
         return self.reachability == ReachabilityState.NORESPONSE