Improve API doc for ArtifactHandler (#1193)

* Rewrite API doc 
Noticed this was a unclear and out of date

maven-artifact/src/main/java/org/apache/maven/artifact/handler/ArtifactHandler.java

@@ -19,10 +19,10 @@
 package org.apache.maven.artifact.handler;
 
 /**
- * An artifact handler defines for a dependency type, defined as Plexus role:<ul>
- * <li>extension and classifier, to be able to download the file,</li>
- * <li>information on how to use the artifact: whether to add it to the classpath, or to take into account its
- * dependencies.</li>
+ * An artifact handler contains information explaining how an artifact plugs into the Maven build:<ul>
+ * <li>Information needed to find the artifact file in a repository including extension and classifier</li>
+ * <li>Information on how to use the artifact as a dependency: whether to add it to the classpath, whether to load its
+ * dependencies transitively</li>
  * </ul>
  *
  * @author <a href="mailto:jason@maven.org">Jason van Zyl</a>
@@ -32,7 +32,8 @@ public interface ArtifactHandler {
     String ROLE = ArtifactHandler.class.getName();
 
     /**
-     * Get the file extension associated to the file represented by the dependency type.
+     * Returns the file name extension of the artifact;
+     * e.g. "jar", "pom", "xml", etc.
      *
      * @return the file extension
      */
@@ -41,7 +42,7 @@ public interface ArtifactHandler {
     String getDirectory();
 
     /**
-     * Get the classifier associated to the dependency type.
+     * Returns the default classifier used if a different one is not set in pom.xml.
      *
      * @return the classifier
      */