Add a comment to offer a hint to an explanation for why we have a unique constraint on (app_id, pushkey, user_id)

synapse/storage/schema/delta/30/deleted_pushers.sql

@@ -18,6 +18,7 @@ CREATE TABLE IF NOT EXISTS deleted_pushers(
     app_id TEXT NOT NULL,
     pushkey TEXT NOT NULL,
     user_id TEXT NOT NULL,
+    /* We only track the most recent delete for each app_id, pushkey and user_id. */
     UNIQUE (app_id, pushkey, user_id)
 );