Update allowed-values overview

As part of #411, use IETF BCP-14 languages as used
with other parts of the specification. Change the wording to address how
implementers would follow constraint directives in their processor, and
align example to be like computer model themes with tutorials and other
documentation.

website/content/specification/syntax/constraints.md

@@ -14,27 +14,25 @@ TODO: P3: Address issue https://github.com/usnistgov/metaschema/issues/325
 
 ## Enumerated values
 
-Additionally, flags may be constrained to a set of known values listed in advance.
+One kind of Metaschema constraint is restricting assembly, field, or flag value(s) with `allowed-values` enumerations. Metaschema processors MUST process `allowed-values` enumerations as defined with the `allow-other` attribute.
 
-This restriction can be either:
+1. When the `allow-other` attribute is defined as `allow-other="no"`, the processor MUST strictly validate content instances with enumerations: only defined `enum` values are valid for the given target(s).
+2. When the `allow-other` attribute is defined as `allow-other="yes"`, the processor MUST loosely validate content instances with enumerations: both `enum` values and other values are valid.
+3. When the `allow-other` attribute is not explicitly defined, the Metaschema processor MUST strictly validate content instances. The implied default is `allow-other="no"`.
 
-1. strict (values must be in the list for document validity with `allow-other="no"` attribute for an `allowed-values` element) or
-2. loose (i.e. for documentation only, no effect in schemas, with `allow-other="yes"`).
-
-If an `allowed-values` constraint does not have the `allow-other` attribute defined, the default is `allow-other="no"`, resulting in strict validation where the only valid values are those in the list.
-
-Within `allowed-values` of a `constraint`, an `enum` element's `@value` attribute assigns the permissible value, while its data content provides documentation. For example:
+Within `allowed-values` of a `constraint`, a Metaschema processor MUST strictly or loosely validate `enum` values with the `@value` attribute. A Metaschema processor MAY use the text value of the `enum`'s XML element as documentation for a given allowed value enumeration. Below is an example.
 
 ```xml
-<define-flag name="algorithm" datatype="string">
-    <formal-name>Hash algorithm</formal-name>
-    <description>Method by which a hash is derived</description>
+<define-flag name="form-factor">
+  <formal>Computer Form Factor</formal-name>
+    <description>The type of computer in the example application's data model.</description>
     <constraint>
       <allowed-values allow-other="yes">
-        <enum value="SHA-224">Documentation for one permissible option.</enum>
-        <enum value="SHA-256">Documentation for another permissible option.</enum>
+        <enum value="laptop">this text value documents the domain and information model's meaning of a laptop</enum>
+        <enum value="desktop">this text value documents the domain and information model's meaning of a desktop</enum>
       </allowed-values>
-    </constraint> ...
+    </constraint> ...  
+</define-flag>
 ```
 
 ## `define-flag` constraints