extindex: improve comment around git->async_wait_all

I found myself tempted to remove this, but it appears impossible
due to odd messages which have multiple Message-IDs.

diff --git a/lib/PublicInbox/ExtSearchIdx.pm b/lib/PublicInbox/ExtSearchIdx.pm
index 2311161eca99d7a3a3da348b400bc33b38224651..1c2a97588e3432960d87ad50390f20bf35317adf 100644 (file)
--- a/lib/PublicInbox/ExtSearchIdx.pm
+++ b/lib/PublicInbox/ExtSearchIdx.pm
@@ -937,7 +937,9 @@ EOS
                        };
                        $self->git->cat_async($smsg->{blob}, \&dd_smsg, $dd);
                }
-               # need to wait on every single one
+               # need to wait on every single one @smsg contents can get
+               # invalidated inside dd_smsg for messages with multiple
+               # Message-IDs.
                $self->git->async_wait_all;
 
                if (checkpoint_due($sync)) {