clarify that migrating repositories and content also migrates user accounts

A user was recently surprised that Nexus 2 user accounts are also migrated to Nexus 3, even if the "Server Configuration" option was not checked during upgrade.

In fact migrating user accounts like this was deliberate, due to complex interactions that can occur between user accounts and custom roles and privileges possible in Nexus 2.

The book should clarify that migrating user accounts will happen, even if the Server configuration option is not checked.