[2.1] add explanation of why SCO IDs have a predetermined namespace

[sr@ctin.us]

Fwd from https://github.com/oasis-tcs/cti-stix2/issues/168

In draft in section 2.9 of master document it outlines that SCO should  
use uuidv5 and dictates some additional namespace usage. When reading  
this, the first question is "why?? namespace and why UUIDv5" Some  
details should be added or linked to another section that specifically  
explains why SCO has different requirements for the ID/identifier.