Fix outdated documentation regarding References Handling (#786)

Fix #785.

doc/relations-handling.md

@@ -49,9 +49,7 @@ To be able to use this feature, your entities have to match some requirements :
 `@name->property` will call `$name->getProperty()` if ```property``` is not
 public)
 * You can reference private properties [by decorating the property accessor with the `ReflectionPropertyAccessor`](advanced-guide.md#custom-accessor)
-* You can reference entities' ID but you will then have to split fixtures in
-multiple files (this is because objects are persisted at the end of each file
-processing) :
+* You can reference entities' ID :
 
 ```yaml
 # fixture_user.yml
@@ -67,6 +65,9 @@ Nelmio\Entity\Group:
         owner: '@user1->id'
 ```
 
+**Warning:** If you are using IDs this way, this either means you are setting the IDs when your object is created like
+with UUIDs or you will not have any guarantee the ID is not already used in the database.
+
 If you want to create ten users and ten groups and have each user own one
 group, you can use `<current()>` which is replaced with the current ID of
 each iteration when using fixture ranges: