Add some missing words of explanation

Because apparently I can't type English.

src/textual/widgets/_option_list.py

@@ -571,7 +571,8 @@ def _duplicate_id_check(self, candidate_items: list[OptionListContent]) -> None:
         }
         # Get the set of new IDs that we're being given.
         new_option_ids = set(option.id for option in new_options)
-        # Now check for duplicates, both internally amongst the
+        # Now check for duplicates, both internally amongst the new items
+        # incoming, and also against all the current known IDs.
         if len(new_options) != len(new_option_ids) or not new_option_ids.isdisjoint(
             self._option_ids
         ):