PEP-654: warn that leaf_generator recycles the tracebacks list (GH-2109)

pep-0654.rst

@@ -469,7 +469,12 @@ recursively, as follows:
         else:
             # exc is a leaf exception and its traceback
             # is the concatenation of the traceback
-            # segments in tbs
+            # segments in tbs.
+
+            # Note: the list returned (tbs) is reused in each iteration
+            # through the generator. Make a copy if your use case holds
+            # on to it beyond the current iteration or mutates its contents.
+
             yield exc, tbs
         tbs.pop()