Clarify behavior for multiple constraints

A key part of usnistgov#411 is clarifying what happens in the
presence of multiple allowed-values constraints. This commit adds
wording to address that.

website/content/specification/syntax/constraints.md

@@ -35,8 +35,12 @@ Within `allowed-values` of a `constraint`, a Metaschema processor MUST strictly
 </define-flag>
 ```
 
-## `define-flag` constraints
+### `define-flag` constraints
 
-## `define-field` constraints
+### `define-field` constraints
 
-## `define-assembly` constraints
+### `define-assembly` constraints
+
+### Multiple allowed value constraints for the same target
+
+A Metaschema processor MUST process and support validation of content instances of Metaschema modules with multiple `allowed-values` constraints for the same targets with an implicit or explicit `allow-other="no"` directive. Fields and flags may have their own internal enumerations, and assemblies themselves can define additional `allowed-values` constraints with `target` attributes for context-sensitive constraints to the same fields and flags, the result is compositional. A Metaschema processor MUST validate content instances with these constraints when there is an explicit or implicit `allow-other="no"` as a union of all enumerations applicable to the context-sensitive target. Below is an example.