perldelta.pod: describe the bug where "0+" methods were sometimes called in reverse order

The previous commit "pp.c, pp_hot.c: Reorder SvXV_nomg() to fetch operands
in left-to-right order" will not only change the order of "Use of
uninialized value" warnings, but actually also change the order of
the call of "0+" overloading methods for each operands.

Author: t-a-k

pod/perldelta.pod

@@ -376,6 +376,17 @@ manager will later use a regex to expand these into links.
 
 XXX
 
+=item *
+
+When both operands of arithmetic operators (C<+>, C<->, etc.) are
+L<overload>ed objects which have no method for that operator but have
+a C<0+> method and the C<fallback> option set to TRUE,
+perl will normally call the C<0+> method for the left operand first
+and then call one for the right operand, i.e. in the same order
+with operand evaluation order.
+But if C<S<use integer;>> is in effect, the C<0+> methods used to be
+called in wrong (reverse) order.
+
 =back
 
 =head1 Known Problems