Clarify shared_bucket_access and import_docs config options

We've received feedback that the docs aren't clear about a few aspects of shared_bucket_access and import_docs:

The docs aren't quite clear about when/where to set each one so it might just be helpful to say more explicitly that shared_bucket_access needs to be 'true' on all nodes participating in such a configuration whereas import_docs can only be set to 'true' on a single node.