add a comment about lack of migrations

Author: Oleg Komarov

models/user/spamreport.go

@@ -56,6 +56,10 @@ func (*SpamReport) TableName() string {
 }
 
 func init() {
+	// This table doesn't exist in the upstream code.
+	// We don't introduce migrations for it to avoid migration id clashes.
+	// Gitea will create the table in the database during startup,
+	// so no manual action is required until we start modifying the table.
 	db.RegisterModel(new(SpamReport))
 }