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 thange the order of
the call of "0+" overloading methods for each operands.

Author: t-a-k

pod/perldelta.pod

@@ -376,6 +376,16 @@ 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
+C<0+> method with C<fallback> option being set to TRUE,
+perl will call the C<0+> methods for these operands in the same order
+with operand evaluation order, i.e. left-to-right normally,
+but if S<C<use integer;>> is in effect they used to be called
+in reverse order.
+
 =back
 
 =head1 Known Problems