Add precision about behavior of get op. when identifier changes

[welcoMattic]

Considering this code

#[ApiProperty(identifier: false)]
public $id;

#[ApiProperty(identifier: true)]
public $code;

The get operation will now expect code value to retrieve the resource.

cc @soyuka

[vinceAmstoutz]

Thanks @welcoMattic!

[vinceAmstoutz]

After a check, seems to be not relevant anymore