[MNG-5771] enabled modello xdoc/xsd for core extensions mdo, with

minimum intro.
Still need to improve descriptions and info in mdo

maven-embedder/pom.xml

@@ -134,23 +134,10 @@
         <artifactId>modello-maven-plugin</artifactId>
         <configuration>
           <version>1.0.0</version>
-        </configuration>
-        <executions>
-          <execution>
-            <id>standard</id>
-            <configuration>
           <models>
             <model>src/main/mdo/core-extensions.mdo</model>
           </models>
         </configuration>
-          </execution>
-          <execution>
-            <id>site-docs</id>
-            <configuration>
-              <models/>
-            </configuration>
-          </execution>
-        </executions>
       </plugin>
     </plugins>
   </build>

maven-embedder/src/main/mdo/core-extensions.mdo

@@ -20,9 +20,17 @@
 
 -->
 
-<model xmlns="http://modello.codehaus.org/MODELLO/1.4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
-  <id>CoreExtensions</id>
-  <name>CoreExtensions</name>
+<model xmlns="http://modello.codehaus.org/MODELLO/1.4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://modello.codehaus.org/MODELLO/1.4.0 http://modello.codehaus.org/xsd/modello-1.4.0.xsd"
+  xml.namespace="http://maven.apache.org/EXTENSIONS/${version}"
+  xml.schemaLocation="http://maven.apache.org/xsd/core-extensions-${version}.xsd">
+
+  <id>core-extensions</id>
+  <name>Core Extensions</name>
+  <description><![CDATA[
+  <p>This is a reference for the Core Extensions descriptor.</p>
+  <p>The default location for the Core Extensions descriptor file is <code>${maven.projectBasedir}/.mvn/extensions.xml</code></p>
+  ]]></description>
 
   <defaults>
     <default>
@@ -32,12 +40,14 @@
   </defaults>
 
   <classes>
-    <class rootElement="true" xml.tagName="extensions">
+    <class rootElement="true" xml.tagName="extensions" xsd.compositor="sequence">
       <name>CoreExtensions</name>
+      <description>Extensions to load.</description>
       <version>1.0.0+</version>
       <fields>
         <field>
           <name>extensions</name>
+          <description>A set of build extensions to use from this project.</description>
           <version>1.0.0+</version>
           <association xml.itemsStyle="flat">
             <type>CoreExtension</type>
@@ -48,22 +58,26 @@
     </class>
     <class xml.tagName="extension">
       <name>CoreExtension</name>
+      <description>Describes a build extension to utilise.</description>
       <version>1.0.0+</version>
       <fields>
         <field>
           <name>groupId</name>
+          <description>The group ID of the extension's artifact.</description>
           <version>1.0.0+</version>
           <required>true</required>
           <type>String</type>
         </field>
         <field>
           <name>artifactId</name>
+          <description>The artifact ID of the extension.</description>
           <version>1.0.0+</version>
           <required>true</required>
           <type>String</type>
         </field>
         <field>
           <name>version</name>
+          <description>The version of the extension.</description>
           <version>1.0.0+</version>
           <required>true</required>
           <type>String</type>