Improve descriptions for EvidenceVariable.characteristic.timeFromEvent

The EvidenceVariable.characteristic.timeFromEvent is described as 'Observation time from study specified event.' but may not be specific to a 'study'.   Please change to 'Timing in which the characteristic is determined.'  Also change descriptions for the contained quantity and range elements which currently include 'after the study start' as the use is now many timeFromEvent concepts and no longer timeFromStart.