Docs: explicitly document why get_filler_item_name may return non-IC.filler items, despite its name

worlds/AutoWorld.py

@@ -484,7 +484,12 @@ def create_item(self, name: str) -> "Item":
         raise NotImplementedError
 
     def get_filler_item_name(self) -> str:
-        """Called when the item pool needs to be filled with additional items to match location count."""
+        """Called when the item pool needs to be filled with additional items to match location count.
+
+        The returned item name must be for a "repeatable" item, i.e. one that it's okay to generate arbitrarily many of.
+        For most worlds this will be one or more of your ItemClassification.filler items, but despite the method name,
+        that's not a requirement: trap, useful and even progression items are valid as long as they're repeatable.
+        Likewise, a unique/non-repeatable filler item would be a poor choice here."""
         logging.warning(f"World {self} is generating a filler item without custom filler pool.")
         return self.random.choice(tuple(self.item_name_to_id.keys()))