Fix docs for getErrorMessagesIndexedByAttribute - array keys must be strings

[arogachev]

#669 was merged incorrectly.

[what-the-diff]

PR Summary

• Updated Error Documentation
  The section in the guide referencing error handling was improved to help developers understand and manage errors more effectively. The following changes were made:
  • Clarified the necessity of using string attribute names with Each.
  • Supplied an example of the InvalidArgumentException that occurs when non-string keys are used for top-level attributes.
  • Illustrated a valid use-case of an array with string keys in conjunction with the Each rule.
  • Enhanced explanation on indexed error messages by path and offered an alternative method for custom separators.
  • Included a guide on how to access the original, unaltered list of error objects.
  • Defined the data content in each error object.
  • Highlighted the omission of a built-in feature to generate a nested tree of error messages.
  • Refreshed the reference link to tips about using keys with separators or shortcuts. This enhances the overall understanding of error handling in different scenarios.