Add javadoc

pf4j/src/main/java/org/pf4j/processor/ExtensionAnnotationProcessor.java

 import java.util.TreeSet;
 
 /**
+ * Processes {@link Extension} annotations and generates an {@link ExtensionStorage}.
+ * You can specify the concrete {@link ExtensionStorage} via processor's environment options
+ * ({@link ProcessingEnvironment#getOptions()}) or system property.
+ * In both variants the option/property name is {@code pf4j.storageClassName}.
+ *
  * @author Decebal Suiu
  */
 public class ExtensionAnnotationProcessor extends AbstractProcessor {
 
         return storage;
     }
+
 }

pf4j/src/main/java/org/pf4j/processor/ExtensionStorage.java

 import java.util.Set;
 
 /**
+ * It's a storage (database) that persists {@link org.pf4j.Extension}s.
+ * The standard operations supported by storage are {@link #read} and {@link #write}.
+ * The storage is populated by {@link ExtensionAnnotationProcessor}.
+ *
  * @author Decebal Suiu
  */
 public abstract class ExtensionStorage {

pf4j/src/main/java/org/pf4j/processor/LegacyExtensionStorage.java

 import java.util.regex.Pattern;
 
 /**
+ * Stores {@link org.pf4j.Extension}s in {@code META-INF/extensions.idx}.
+ *
  * @author Decebal Suiu
  */
 public class LegacyExtensionStorage extends ExtensionStorage {

pf4j/src/main/java/org/pf4j/processor/ServiceProviderExtensionStorage.java

 import java.util.regex.Pattern;
 
 /**
+ * Stores {@link org.pf4j.Extension}s in {@code META-INF/services}.
+ *
  * @author Decebal Suiu
  */
 public class ServiceProviderExtensionStorage extends ExtensionStorage {