Update definition of Session for Data Standard 3.x

Under 3.0, termDescriptor was removed from the Session key in favor of a Session name. I believe this design change in part was motivated by a desire for termDescriptors to be generic, as in

• "semester"
• "quarter"
• etc.

Where under DS 2.x, those termDescriptors forced an enumeration of all sessions, as in:

• "first semester"
• "second semester"
• etc.

However, the definition of termDescriptor for 3.0 model was not updated and provides an example consistent with 2.0 usage:

An descriptor value indicating the term (e.g. 'Fall Semester’, 'Second Quarter’, 'Summer Semester’, etc.).

Should the definition be changed?