Clarify definitions of StudentIdentificationSystem

In rostering field work, it has become clear that there are generally 3 possible IDs for students in play:

1. A student information system identifier, which is the ID generally assigned by the SIS system and used in most district operations to join together various data on the student. In Clever rostering, this corresponds to sis_id
2. A state identification number, which is the ID assigned by the state and similarly used to join together various data within the state context. In Clever rostering, this is the state_id
3. A local identifier often provided to students directly to remember and use in the students daily interactions. This is sometimes referred to as the "student number" (Clever "student_number"). It may be the same as the student information system identifier.

These should be clarified in descriptions of the IDs in the default StudentIdentificationSystem descriptors.