Flow delete offers to delete its runs, history included

DELETE /runs?flow= removes every Run row of the flow but left its flow_run
rows, so a deleted flow kept appearing in the history. history=true now
drops those too, and the flow delete dialogs (editor and bulk) send it when
"Also delete its runs" is ticked.

docs/concepts/runs.md

@@ -191,9 +191,11 @@ Select two or more runs to compare curves and the parameters that differ.
 Plot against step, elapsed seconds or another metric from the same runs.
 Comparisons have shareable URLs.
 
-Deleting a flow keeps its run history. You can still inspect those runs, but
-cannot retry them until the flow exists again. Delete history separately,
-after active runs finish or are cancelled.
+Deleting a flow keeps its run history, and the artifacts those runs recorded
+stay on disk. You can still inspect those runs, but cannot retry them until
+the flow exists again. Tick **Also delete its runs** in the delete dialog to
+clear them with the flow, or delete history separately after active runs
+finish or are cancelled.
 
 ### Taking it into a dataframe