Documentation : improved Copy task VS copy() method area

Documentation improvement in the area of the Copy task VS copy() method (16.6. Copying files).

Related to forum thread : http://forums.gradle.org/gradle/topics/documentation_improvement_for_copy_task_vs_project_copy_method

FYI I've tested the example changes with : 
<pre><code>gradlew integTest:integTest -D:integTest:integTest.single=UserGuideSamplesIntegrationTest -Dorg.gradle.userguide.samples.filter=userguide/files/copy</code></pre>

If you intend to merge it, let me know, I'll send the CLA.