Add note about topicalizing rhs for infix:<andthen>.

Author: pmichaud

lib/Language/operators.pod

@@ -1345,7 +1345,9 @@ otherwise the last operand. Short-circuits.
 =head2 infix C«andthen»
 
 Returns the first undefined argument, otherwise the last argument.
-Short-circuits.
+Short-circuits.  The result of the left side is bound to $_ for the
+right side, or passed as arguments if the right side is a block or
+pointy block.
 
 =head1 Loose OR Precedence