Document preconditions for iterating over GeneralQueryResults.

GeneralQueryResults.iter_tuple (and DatasetAssociation.from_query_result, which
calls it) assume dataset_id and run columns for the dataset type(s) of
interest. However, these columns do not exist unless the query call
specifically asks for them.

python/lsst/daf/butler/_dataset_association.py

@@ -72,10 +72,11 @@ def from_query_result(
 
         Parameters
         ----------
-        result : `GeneralQueryResults`
-            General query result returned by `Query.general` method. The result
-            has to include "{dataset_type.name}.timespan" and
-            "{dataset_type.name}.collection" columns.
+        result : `~lsst.daf.butler.queries.GeneralQueryResults`
+            General query result returned by
+            `Query.general <lsst.daf.butler.queries.Query.general>` method. The
+            result has to include "dataset_id", "run", "collection", and
+            "timespan" dataset fields for ``dataset_type``.
         dataset_type : `DatasetType`
             Dataset type, query has to include this dataset type.
         """

python/lsst/daf/butler/queries/_general_query_results.py

@@ -111,6 +111,9 @@ def iter_tuples(self, *dataset_types: DatasetType) -> Iterator[GeneralResultTupl
         """Iterate over result rows and return data coordinate, and dataset
         refs constructed from each row, and an original row.
 
+        This object has to include "dataset_id" and "run" columns for each type
+        in ``dataset_types``.
+
         Parameters
         ----------
         *dataset_types : `DatasetType`