Update documentation and add constraints to EventDuration field for StudentSchoolAttendanceEvent

Per DATASTD-1857, update documentation and add constraints to EventDuration field for StudentSchoolAttendanceEvent, as follows:

1. Update the documentation to be "The amount of time as a decimal fraction for the event as recognized by the school: 1 day = 1.0, 1/2 day = 0.5, 1/3 day = 0.33. The value can have up to two decimal places with a min of 0.00 and max of 1.00."
2. Add min value constraint of 0.0 and max value constraint of 1.0 to the API