Clearer explanation AWS_CUSTOM_DOMAIN + protocol

This is a really small MR to improve documentation about object storage. django-storage offers an AWS_S3_CUSTOM_DOMAIN variable to change the domain used to serve objects, while still using the S3 endpoints for changes etc. It was imho not really clear in the documentation.

I added the AWS_S3_URL_PROTOCOL from django-storage because someone could want to e.g. disable encryption for the custom domain.

This is my first MR, please tell if I did something wrong.

Thanks for this amazing software ! 😄