Document Connection::afterCommit() and deferred commit events

[jamisonbryant]

Summary

Documentation for Connection::afterCommit() and the updated behavior of afterSaveCommit/afterDeleteCommit events introduced in cakephp/cakephp#19383.

Major Changes

• Added new afterCommit section to database-basics.md documenting Connection::afterCommit() with API signature, usage examples (basic and nested transactions), and explanation of immediate-execution semantics outside transactions
• Updated afterSaveCommit section in table-objects.md to document the new deferred behavior when called inside outer transactions, replacing the previous "not triggered" language
• Updated afterDeleteCommit section in table-objects.md with the same deferred behavior documentation
• Added Changed in version 5.4.0 annotations to both event sections
• Added entries to the 5.4 migration guide under Behavior Changes (ORM) and New Features (Database)

Minor Changes

• Fixed typo in afterDeleteCommit description ("has been is committed" -> "has been committed")

Work Remaining

None.

Test Plan

• Verify database-basics.html#aftercommit renders correctly with code examples and version annotation
• Verify table-objects.html#aftersavecommit shows updated behavior description and version annotation
• Verify table-objects.html#afterdeletecommit shows updated behavior description and version annotation
• Verify 5.4 migration guide lists the behavior change and new feature