Explain rates rules kinds more explicitly

[Fredar]

• What exactly are they for, best with examples and how they use variables.
• Give a mapping of possible variables

additional_person_fixed_per_night => [:occupation_greater_than]
arrival_only                      => [:days]
charge_at_least                   => [:length, :unit]
charge_at_least_if_available      => [:length, :unit]
departure_only                    => [:days]
full_period_only                  => []
prevent_if_booked_ahead_of        => [:length, :unit]
prevent_if_booked_less_than       => [:length, :unit]
strict_minimum_price_per_night    => []
weekend_night                     => [:days]
stay_at_least                     => [:length, :unit]
stay_shorter_than                 => [:length, :unit]
early_booking                     => [:length, :unit]
late_booking                      => [:length, :unit]
occupation_less_than              => [:occupation_less_than]