Clarify the impact of changing the fetch mode

Seems to work for one-to-many as well, but with a negative performance impact.

docs/en/reference/dql-doctrine-query-language.rst

@@ -935,7 +935,8 @@ the Query class. Here they are:
 
 -  ``Query#getResult()``: Retrieves a collection of objects. The
    result is either a plain collection of objects (pure) or an array
-   where the objects are nested in the result rows (mixed).
+   where the objects are nested in the result rows (
+   ).
 -  ``Query#getSingleResult()``: Retrieves a single object. If the
    result contains more than one object, an ``NonUniqueResultException``
    is thrown. If the result contains no objects, an ``NoResultException``
@@ -1380,7 +1381,13 @@ Given that there are 10 users and corresponding addresses in the database the ex
     SELECT * FROM address WHERE id IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
 
 .. note::
-    Changing the fetch mode during a query is only possible for one-to-one and many-to-one relations.
+    Changing the fetch mode during a query mostly makes sense for one-to-one and many-to-one relations. In that case, 
+    all the necessary IDs are available after the root entity (``user`` in the above example) has been loaded. So, one
+    query per association can be executed to fetch all the referred-to entities (``address``).
+    
+    For one-to-many relations, changing the fetch mode to eager will cause to execute one query **for every root entity
+    loaded**. This gives no improvement over the ``lazy`` fetch mode which will also initialize the associations on
+    a one-by-one basis once they are accessed.
 
 
 EBNF