Document Google Cloud Storage visibility handler

[jannes-io]

I had to dig through both league/flysystem-bundle and league/flysystem-google-cloud-storage code to figure out why I am getting an error regarding ACLs in a uniform access bucket and how I can use the visibility handlers.

Please provide more documentation regarding the visibility_handler option in the adapter extension. There's not even a mention of it anywhere.

Thanks.

[maxhelias]

The original PR is here : #160

If anyone wants to take care of it, the mention is indeed missing on these pages :

• https://github.com/thephpleague/flysystem-bundle/blob/3.x/docs/2-cloud-storage-providers.md#google-cloud-storage
• https://github.com/thephpleague/flysystem-bundle/blob/3.x/docs/B-configuration-reference.md