document supported values

Author: hboutemy

api/maven-api-model/src/main/mdo/maven.mdo

@@ -92,7 +92,8 @@
           <name>modelVersion</name>
           <version>4.0.0+</version>
           <required>true</required>
-          <description>Declares to which version of project descriptor this POM conforms.</description>
+          <description>Declares to which version of project descriptor this POM conforms to:
+            {@code 4.0.0} for Maven 3 or {@code 4.1.0} for Maven 4.</description>
           <type>String</type>
         </field>