minor #9259 Explaining the "one-to-many" case more explicitly (ThomasLandauer)

This PR was squashed before being merged into the 2.7 branch (closes #9259).

Discussion
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Explaining the "one-to-many" case more explicitly

Please double-check: I'm not certain if "For a many-to-many relationship:" (line 574) is correct!

Commits
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5636b52 Explaining the "one-to-many" case more explicitly

Author: javiereguiluz

form/form_collections.rst

@@ -561,15 +561,19 @@ you will learn about next!).
 
         // src/AppBundle/Entity/Task.php
 
-        // ...
         public function addTag(Tag $tag)
         {
+            // for a many-to-many association:
             $tag->addTask($this);
 
+            // for a many-to-one association:
+            $tag->setTask($this);
+
             $this->tags->add($tag);
         }
 
-    Inside ``Tag``, just make sure you have an ``addTask()`` method::
+    If you're going for ``addTask()``, just make sure you have an appropriate method
+    that looks something like this::
 
         // src/AppBundle/Entity/Tag.php
 
@@ -581,9 +585,6 @@ you will learn about next!).
             }
         }
 
-    If you have a one-to-many relationship, then the workaround is similar,
-    except that you can simply call ``setTask()`` from inside ``addTag()``.
-
 .. _form-collections-remove:
 
 Allowing Tags to be Removed