Update "NEEDS DOCUMENTATION" placeholder to provide appropriate documentation

Summary of outcomes for Data Standard 2.1

• Problem Addressed: Several "NEEDS DOCUMENTATION" placeholders still exist in model, awaiting appropriate documentation.
• High-level Reasoning: Appropriate documentation is necessary.
• Changes Made: Added annotation as shown in attached spreadsheet

Original ticket description follows:

MODL-127 replaced "NEEDS DOCUMENTATION" placeholder with proper annotation and made revisions to existing annotation in a number of places in the data standard. The xsd will need to be updated to reflect the new annotation.