How does the QTR-8RC capacitor charging works?
The third pint should read  


  Allow at least 10 us for the 10 nF capacitor to discharge.


If the capacitor has Vcc on both sides it's discharged. (IR) light on the phototransistor will cause a current to ground, so that voltage builds up on the capacitor, making its lower side go lower, as the other side is fixed at Vcc. Since the charging is through a current source instead of a resistor the voltage will decrease linearly with time.