docs: require activator in custom actions class name for register

[datamweb]

Description
fix: #1249

In this implementation, since we cannot determine whether the user provides just the class name or the fully qualified namespace of the class, we are unable to directly verify the ActionInterface implementation. Therefore, we rely on the class name suffix ('Activator') as a consistent identifier for validation.

see:

shield/src/Traits/Activatable.php

Line 63 in 58d6d6f

return str_contains(setting('Auth.actions')['register'] ?? '', 'Activator');

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[datamweb]

Thanks!