fix doc about filter subroutines' $_[0]

Filter subroutines established by an @inc callback have always received
a numeric zero as their first argument, not a referencet to the sub
as documented.  Fixes [perl #115754].

pod/perlfunc.pod

@@ -6507,11 +6507,12 @@ subroutine will be called to act as a simple source filter, with the
 line as read in L<C<$_>|perlvar/$_>.
 Again, return 1 for each valid line, and 0 after all lines have been
 returned.
+For historical reasons the subroutine will receive a meaningless argument
+(in fact always the numeric value zero) as C<$_[0]>.
 
 =item 4
 
-Optional state for the subroutine.  The state is passed in as C<$_[1]>.  A
-reference to the subroutine itself is passed in as C<$_[0]>.
+Optional state for the subroutine.  The state is passed in as C<$_[1]>.
 
 =back