Document steps to restore deleted admin account

Added to the existing note under 'Deleting Users'. Also added a line about
ipa user-del and ipa group-remove-member not allowing the last admin user to be
deleted by default.

https://fedorahosted.org/freeipa/ticket/2746

Reviewed-By: Petr Viktorin <pviktori@redhat.com>
Update doc version to 4.0