Improvements to JavaDoc of MongoReader interface.

- clarified restrictions to the implementation

spring-data-mongodb/src/main/java/org/springframework/data/document/mongodb/MongoReader.java

@@ -18,17 +18,20 @@ package org.springframework.data.document.mongodb;
 import com.mongodb.DBObject;
 
 /**
- * A MongoWriter is responsible for converting a native MongoDB DBObject to an object of type T. 
+ * A MongoWriter is responsible for converting a native MongoDB DBObject to an object of type T.
  * 
  * @author Mark Pollack
  * @author Thomas Risberg
+ * @author Oliver Gierke
  *
  * @param <T> the type of the object to convert from a DBObject
  */
 public interface MongoReader<T> {
 
 	/**
-	 * Ready from the native MongoDB DBObject representation to an instance of the class T.
+	 * Ready from the native MongoDB DBObject representation to an instance of the class T. The given type has to be the
+	 * starting point for marshalling the {@link DBObject} into it. So in case there's no real valid data inside
+	 * {@link DBObject} for the given type, just return an empty instance of the given type.
 	 * @param clazz the type of the return value
 	 * @param dbo theDBObject
 	 * @return the converted object