Add Rule 4-15 Clarifying text

[cchipman6]

The following comment was received during the review of the NIEM-based Code List Specification being developed for the NATO Core Data Framework (NCDF):

Specifically for Example D-5 for the complex binding, I wonder whether the elementName attribute should be e.g. “nc:VehicleMakeAbstract”, as that is an element of the nv:VehicleType used for the Vehicle element. And it is the substitution group used for the extension element “ext:VehicleMakeCode”. Looking at Rule 4-15, current wording is not clear whether the value of elementName needs to be an element that is in a substitution group, of is the element that is the base of a substitution group. I recommend clearer rewording.

Proposed solution: Rule 4-15 states that the bound element is the first element in the substitution group named by elementName. Each element is in its own substitution group.
Add text after rule 4-15 clarifying that each element is in its own substitution group, and that elementName may indicate a concrete element or a different element within the substitution group of the named element.