doc lint: emit errors on STDERR

Have all of the scripts invoked by "make check-docs" emit their output
on STDERR. This does not currently matter due to the way we're
invoking them, but will in a subsequent change. It's a good idea to do
this in any case for consistency with other tools we invoke.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/lint-man-section-order.perl

@@ -46,7 +46,7 @@ my $SECTION_RX = do {
 my $exit_code = 0;
 sub report {
 	my ($msg) = @_;
-	print "$ARGV:$.: $msg\n";
+	print STDERR "$ARGV:$.: $msg\n";
 	$exit_code = 1;
 }