Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 bc_valid_username(Arg1)

 bc_unique_username(Arg1)

 bc_unique_username(Arg1, Arg2)

 bc_user_exists(Arg1)

 bc_remaining_admin(Arg1)

 bc_no_entries(Arg1)

 bc_valid_role(Arg1)