docs: add transaction side-effect recipes

[memleakd]

Description

This PR proposes adding practical documentation for deferring side effects with database transaction callbacks.

It expands the transaction callback docs with examples for:

• using afterCommit() for side effects that should only run after committed data is visible
• using afterRollback() for cleanup that should only run when a transaction does not commit
• avoiding assumptions that Model callbacks are automatically commit-aware

Motivation

The afterCommit() and afterRollback() APIs are useful for avoiding subtle consistency issues around database transactions.

For example, application code may need to dispatch a job, trigger an event, clear a cache entry, send a notification, or clean up a temporary file. If that work runs before the surrounding transaction finishes, it may observe data that is not committed yet, or it may run even though the transaction later rolls back.

This documentation gives users a small set of copyable recipes for those cases.

Checklist

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide