DOC Comment on thresholds[0] for roc_curve

Author: jnothman

sklearn/metrics/metrics.py

@@ -845,7 +845,8 @@ class or confidence values.
 
     thresholds : array, shape = [n_thresholds]
         Decreasing thresholds on the decision function used to compute
-        fpr and tpr.
+        fpr and tpr. `thresholds[0]` represents no instances being predicted
+        and is arbitrarily set to `max(y_score) + 1`.
 
     See also
     --------