Revert "Revert "Fix docs for getErrorMessagesIndexedByAttribute - array keys must be strings""

[arogachev]

Reverts yiisoft/validator#673

[what-the-diff[bot]]

PR Summary

• Enhancements in Attribute Rules This PR adds clarity that attribute names should always be in the string format while using the 'Each' rule to prevent misunderstandings or errors.

• Exception Handling Example Update The example code provided has been improved to showcase the scenario in which using non-string keys for top-level attributes can lead to the throwing of an InvalidArgumentException.

• Array Key Formatting Example An extra example has been introduced to help visualise the output when an array with string keys is used.

• Documentation Reference for Special Keys To facilitate user understanding, a hyperlink directing to the documentation for using keys that contain a separator or shortcut has been added.

• PHP Key Type Usage Guide Linked reference to the official PHP documentation to explain how PHP converts keys to the integer type, beneficial for users unfamiliar with this PHP nuance.