Update relations.md (#904)

Update BackLink docs to include the fact that they are virtual.

docs/tutorial/relations.md

@@ -294,8 +294,8 @@ class Person(Document):
 
 The `original_field` parameter is required for the back link field.
 
-Back links support all the operations that normal links support.
+Back links support all the operations that normal links support, but are virtual. This means that when searching the database, you will need to include `fetch_links=True` (see [Finding documents](find.md)), or you will recieve an empty 'BackLink' virtual object. It is not possible to `fetch()` this virtual link after the initial search.
 
 ## Limitations
 
-- Find operations with the `fetch_links` parameter can not be used in the chaning with `delete` and `update` methods.
+- Find operations with the `fetch_links` parameter can not be used in the chaning with `delete` and `update` methods.