Description of the bug

Each constructor has a $label parameter which can be a string or an array.

Currently, docblocks only specify that $label can be a string, making the docblock incorrect.

This causes issues with code quality tools like phpstan.

Steps To Reproduce

1. Set up phpstan on a project with wordpress-fieldmanager
2. Register a new Fieldmanager field, specifying an array for $label, e.g.

new \Fieldmanager_Group(
	[
		'name'     => 'settings',
		'children' => [
			'api_key' => new \Fieldmanager_TextField( __( 'API Key', 'wp-plugin' ) ),
		],
	]
);

4. Run phpstan
5. Notice errors like Parameter #1 $label of class Fieldmanager_Group constructor expects string, array<string, array<string, Fieldmanager_TextField>|string> given.

Additional Information

No response