[MCOMPILER-548] JDK 21 throws annotations processing warning that can…

… not be turned off

Turns out this is just a documentation issue. `full` works fine but is
not documented.

src/main/java/org/apache/maven/plugin/compiler/AbstractCompilerMojo.java

@@ -279,8 +279,11 @@ public abstract class AbstractCompilerMojo extends AbstractMojo {
      * <ul>
      * <li><code>none</code> - no annotation processing is performed.</li>
      * <li><code>only</code> - only annotation processing is done, no compilation.</li>
+     * <li><code>full</code> - annotation processing and compilation.</li>
      * </ul>
      *
+     * <code>full</code> is the default. Starting with JDK 21, this option must be set explicitly.
+     *
      * @since 2.2
      */
     @Parameter