lei_mirror: describe why the {ibx} field is used

I forgot why that hunk of code was needed :x, so maybe others
will find the comment helpful, too.

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/PublicInbox/LeiMirror.pm b/lib/PublicInbox/LeiMirror.pm
index c148ebfd..d878f1e4 100644
--- a/lib/PublicInbox/LeiMirror.pm
+++ b/lib/PublicInbox/LeiMirror.pm
@@ -194,7 +194,7 @@ sub _write_inbox_config {
                 die "open($f): $!";
         }
         my $cfg = PublicInbox::Config->git_config_dump($f, $self->{lei}->{2});
-        my $ibx = $self->{ibx} = {};
+        my $ibx = $self->{ibx} = {}; # for indexing
         for my $sec (grep(/\Apublicinbox\./, @{$cfg->{-section_order}})) {
                 for (qw(address newsgroup nntpmirror)) {
                         $ibx->{$_} = $cfg->{"$sec.$_"};