Clarify documentation of the behavior of a user blocking a domain

[jonpincus]

@ClearlyClaire's github comment here says that when a user blocks a domain, and AUTHORIZED_FETCH is enabled, it prevents the blocked domain from fetching the user's public posts. That's also the behavior I think I saw when I tested it out a couple of weeks ago.

However, the documentation doesn't mention this; it only talks about hiding posts, removing followers, and preventing new followers.