s/Failure/undefined/ for decrement on sets/bags

Author: TimToady

S02-bits.pod

@@ -1695,7 +1695,7 @@ use the C<Hash> interface and increment an element of a C<BagHash>
 its value is increased by one (creating the element if it doesn't exist
 already).  If you decrement the element the value is decreased by one;
 if the value goes to 0 the element is automatically deleted.  An attempt
-to decrement a non-existing value returns a C<Failure> value.  When not
+to decrement a non-existing value returns an undefined value.  When not
 used as a C<Hash> (that is, when used as an C<Array> or list or C<Bag>
 object) a C<BagHash> behaves as a C<Bag> of its keys, with each key
 replicated the number of times specified by its corresponding value.