Document how to enable spring transactional annotations

It's just a matter of adding the following to resources.groovy

```
xmlns tx:"http://www.springframework.org/schema/tx"
tx.'annotation-driven'('transaction-manager':'transactionManager')
```

Start by locating the documentation section that explains Grails configuration and transactional support. Add the provided resources.groovy namespace and annotation-driven configuration, with a brief explanation of enabling Spring transactional annotations. Done means the setup is documented clearly enough for users to apply it.

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