Fix docs for getErrorMessagesIndexedByAttribute - array keys must be strings

[ArduanovDanil]

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Fixed issues	❌

[what-the-diff]

PR Summary

• Clarification in code usage
  This update modified our code examples and error messages to be more user-friendly and precise. They added the word "must" for emphasis, highlighting that attribute names must always be strings.

• Handling of numerical attributes
  Before this update, you could use an array such as [21, 22, 23, 20]. However, this practice will now throw an error because only string-based attributes are accepted. You'll see an error message stating, Top level attributes can only have string type.

• Improved error descriptions for wrong inputs
  When you now use an array with string keys like ['1' => 21, '2' => 22, '3' => 23, '4' => 20], the system will give a more specific error message, ['4' => ['Value must be no less than 21.']], indicating the exact problem and where it's occurring. This will simplify troubleshooting and make errors less daunting.

[arogachev]

@ArduanovDanil Thanks! 👍