Add field and flag a-v constraint explanation.

website/content/specification/syntax/constraints.md

@@ -20,7 +20,9 @@ One kind of Metaschema constraint is restricting assembly, field, or flag value(
 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"`.
 
-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.
+### `define-flag` constraints
+
+An assembly can define `allowed-values` constraints. The constraint MUST have an `allowed-values` values element with one or more `enum` elements for each value subject to strict or loose validation. Each `enum` element MUST have a `value` attribute and MAY have a text value for documenting the allowed value. 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="form-factor">
@@ -35,10 +37,23 @@ Within `allowed-values` of a `constraint`, a Metaschema processor MUST strictly
 </define-flag>
 ```
 
-### `define-flag` constraints
-
 ### `define-field` constraints
 
+A field `allowed-values` constraint is syntactically equivalent to such a constraint for a flag. Syntax and processor requirements are identical. See below for an example.
+
+```xml
+<define-field 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="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> ...  
+</define-field>
+```
+
 ### `define-assembly` constraints
 
 An assembly can define `allowed-values` constraints. Unlike flag and field constraints, assembly `allowed-vaues` constraints have a `target` attribute to indicate the location of element(s) where the enumerated values are applicable for strict or loose validation. A Metaschema processor MUST parse the right-hand side of the `target` attribute, a Metapath expression, to correctly apply strict or loose validation to the relevant elements that match this expression. Below is an example.