DOC: add class docs for RetrieveEntity

src/main/java/org/caosdb/server/entity/RetrieveEntity.java

@@ -24,8 +24,14 @@
  */
 package org.caosdb.server.entity;
 
-// TODO document the use of this class; it seems to exist of only constructors
-
+/**
+ * Entity which is to be retrieved (i.e. read-only).
+ *
+ * <p>This class only exposes those constructors which are necessary for Entity which are to be
+ * retrieved (e.g. Entity(id)) and hide the others (e.g. Entity(XML-Represenation)).
+ *
+ * @author Timm Fitschen <t.fitschen@indiscale.com>
+ */
 public class RetrieveEntity extends Entity {
 
   public RetrieveEntity() {