add issue link for matrix-appservice-irc node override

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jaysa68

flake.nix

@@ -367,6 +367,7 @@
           configureFlags = (old.configureFlags or [ ]) ++ [ "--enable-rpc" ];
         });
 
+        # https://github.com/matrix-org/matrix-appservice-irc/issues/1861
         matrix-appservice-irc = prev.matrix-appservice-irc.override {
           nodejs-slim = final.nodejs-slim_22;
         };