Don't force to write classes for each document type

One of the concept of the documents collection is to allow to drop arbitrary
JSON files and get a PHP object from the dictionary fields.

While it's certainly not optimal, and while code could store the properties
in a specific hashmap instead, this attribute is added for compatibility
with PHP 8.2+.

Ref T2144.

Differential Revision: https://devcentral.nasqueron.org/D4174