[2.1] 6.6.2 inconsistent writing of where rules are found

[sr@ctin.us]

Fwd from: https://github.com/oasis-tcs/cti-stix2/issues/179

[2.1] 6.6.2 inconsistent writing of where rules are found

In 6.6.2 the rule of "body or body_raw_ref must be included" is found  
at the top of the section. In order sections rules such as this are  
found in the specific property descriptions. Suggest we standardize  
here unless there is a reason for not having it in the same place?

@rpiazza then posted:

Other sections (4.11, 4.13, 6.1, 6.7, 6.12) have normative statements  
about multiple property constraints in the text, but only 6.1 and 4.13  
also has something in the property table. I wonder if that was because  
in 6.1 and 4.13, the constraint is for one or the other, but not both.  
@jordan2175 - do you remember if we did this on purpose?