Add more doc for disabled transactions

Author: Philippe Lafoucrière

driver/postgres/README.md

@@ -27,5 +27,10 @@ Since all migrations are executed in a transaction block by default (per migrati
 -- disable_ddl_transaction
 alter type ...;
 ```
+
 The option `disable_ddl_transaction` must be in a sql comment of the first line of the migration file.
-If set, the driver will execute the file content directly.
+
+Please note that you can't put several `alter type ... add value ...` in a single file. Doing so will result in a `ERROR 25001: ALTER TYPE ... ADD cannot be executed from a function or multi-command string` sql exception during migration.
+
+Since the file will be executed without transaction, it's probably not a good idea to exec more than one statement anyway. If the last statement of the file fails, chances to run again the migration without error will be very limited.
+