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

[arogachev]

Reverts #669

[what-the-diff]

PR Summary

• Enforcing Required Attribute Names as Strings
  Previously, the attribute names would accept any data type when used with the Each rule. Going forward, only string types will be accepted as attribute names for each rule, as per this update.

• Updated Array Input Exception Understanding
  In older versions, the use of non-string keys with top level attributes (like [21, 22, 23, 20]) could result in throwing InvalidArgumentException. This pull request modifies this behaviour inherently by changing such an example from a potential error source to a valued input, producing an output like ['1' => ['Value must be no less than 21.'], '2' => ['Value must be no less than 21.'],].

• Removal of Specific Documentation Link
  The link dedicated to documentation about usage of keys containing separators or shortcuts, has been removed in this pull request. Please refer to the updated documentation for recent understanding of keys and shortcuts.