title: clarification on behaviour of merge
body: <p>Suppose I have an object A with id 1 on my persistence layer (i.e. on the DB)</p>
  
  <p>If I do the following</p>
  
  <pre><code>A a = new A();
  a.setId(1);
  a.setSomeField("a value");
  A managed_a = entityManager.merge(a);
  </code></pre>
  
  <p>would the entity manager retrieve the persisted entity with id 1 on the DB,
  make it managed, and update it accordingly ?</p>
  
  <p>Or the object with id 1 has to exsist in the persistence context in order in order for the above to happen ?</p>