p.haul: explain how to prepare p.haul to work, v2

Currently our docs do not explain how to prepare p.haul to
work.

Reported-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>